drm/i915: Select CONFIG_SHMEM

drm/i915: Select CONFIG_SHMEM

commit ca9ab10033d190c1ede85fdf456307bdfdabf079 upstream.

The driver requires shmfs as the backing filesystem to handle the buffer
objects, so ensure it is selected if the user chooses to build our
driver.

Fixes: Bug 14662 - Dell E5500 kernel panic with KMS
http://bugzilla.kernel.org/show_bug.cgi?id=14662

The revealing nature of the panic is the NULL function pointer
dereference in read_cache_page_async().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Mateusz Kaduk <mateusz.kaduk@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>