drm/i915: Add dependency on CONFIG_TMPFS

drm/i915: Add dependency on CONFIG_TMPFS

Without tmpfs, shmem_readpage() is not compiled in causing an OOPS as
soon as we try to allocate some swappable pages for GEM.

Jan 19 22:52:26 harlie kernel: Modules linked in: i915(+) drm_kms_helper cfbcopyarea video backlight cfbimgblt cfbfillrect
Jan 19 22:52:26 harlie kernel:
Jan 19 22:52:26 harlie kernel: Pid: 1125, comm: modprobe Not tainted 2.6.37Harlie #10 To be filled by O.E.M./To be filled by O.E.M.
Jan 19 22:52:26 harlie kernel: EIP: 0060:[<00000000>] EFLAGS: 00010246 CPU: 3
Jan 19 22:52:26 harlie kernel: EIP is at 0x0
Jan 19 22:52:26 harlie kernel: EAX: 00000000 EBX: f7b7d000 ECX: f3383100 EDX: f7b7d000
Jan 19 22:52:26 harlie kernel: ESI: f1456118 EDI: 00000000 EBP: f2303c98 ESP: f2303c7c
Jan 19 22:52:26 harlie kernel:  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Jan 19 22:52:26 harlie kernel: Process modprobe (pid: 1125, ti=f2302000 task=f259cd80 task.ti=f2302000)
Jan 19 22:52:26 harlie kernel: Stack:
Jan 19 22:52:26 harlie udevd-work[1072]: '/sbin/modprobe -b pci:v00008086d00000046sv00000000sd00000000bc03sc00i00' unexpected exit with status 0x0009
Jan 19 22:52:26 harlie kernel:  c1074061 000000d0 f2f42b80 00000000 000a13d2 f2d5dcc0 00000001 f2303cac
Jan 19 22:52:26 harlie kernel:  c107416f 00000000 000a13d2 00000000 f2303cd4 f8d620ed f2cee620 00001000
Jan 19 22:52:26 harlie kernel:  00000000 000a13d2 f1456118 f2d5dcc0 f1a40000 00001000 f2303d04 f8d637ab
Jan 19 22:52:26 harlie kernel: Call Trace:
Jan 19 22:52:26 harlie kernel:  [<c1074061>] ? do_read_cache_page+0x71/0x160
Jan 19 22:52:26 harlie kernel:  [<c107416f>] ? read_cache_page_gfp+0x1f/0x30
Jan 19 22:52:26 harlie kernel:  [<f8d620ed>] ? i915_gem_object_get_pages+0xad/0x1d0 [i915]
Jan 19 22:52:26 harlie kernel:  [<f8d637ab>] ? i915_gem_object_bind_to_gtt+0xeb/0x2d0 [i915]
Jan 19 22:52:26 harlie kernel:  [<f8d65961>] ? i915_gem_object_pin+0x151/0x190 [i915]
Jan 19 22:52:26 harlie kernel:  [<c11e16ed>] ? drm_gem_object_init+0x3d/0x60
Jan 19 22:52:26 harlie kernel:  [<f8d65aa5>] ? i915_gem_init_ringbuffer+0x105/0x1e0 [i915]
Jan 19 22:52:26 harlie kernel:  [<f8d571b7>] ? i915_driver_load+0x667/0x1160 [i915]

Reported-by: John J. Stimson-III <john@idsfa.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org