drm/i915: Tune down init error message due to failure injection

drm/i915: Tune down init error message due to failure injection

Atm, in case failure injection forces an error the subsequent "*ERROR*
failed to init modeset" error message will make automated tests (CI)
report this event as a breakage even though the event is expected. To
fix this print the error message with debug log level in this case.

While at it print the error message for any init failure and change it
to
"""
Device initialization failed (errno)
Please file a bug at https://bugs.freedesktop.org/enter_bug.cgi?product=DRI
against DRM/Intel providing the dmesg log by booting with drm.debug=0xf
"""
and export a helper printing error messages using this same format.
A follow-up patch will convert all uses of DRM_ERROR reporting a user
facing problem to use this new helper instead.

v2:
- Include the problematic error message in the commit log, add a
  request to file an fdo bug to the message (Chris)
v3:
- Include the new error message too in the commit log, make the
  fdo link more precise and print part of the message with info log
  level (Chris)
v4: (Chris)
- Use dev_printk instead of DRM_ERROR/INFO and use NOTICE instead of
  INFO loglevel
- Export a helper for printing user facing error messages
v5:
- Keep the DRM_ERROR message prefix used by piglit-igt/CI to filter
  relevant dmesg lines
- Use dev_notice(), instead of dev_printk(KERN_NOTICE,...)
v6:
- Print the fdo bug link only once (Chris)

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458290770-15480-1-git-send-email-imre.deak@intel.com