drm: move drm_can_sleep() to drm_util.h

Move drm_can_sleep() out of drmP.h to allow users
to get rid of the drmP.h include.

There was no header file that was a good match for this helper function.
So add this to drm_util with the relevant includes.

Add include of drm_util.h to all users.

v2:
- Update comments to use kernel-doc style (Daniel)
- Add FIXME to drm_can_sleep and add note that this
  function should not be used in new code (Daniel)

v3:
- Fix kernel-doc syntax (Daniel)
- Plug drm_util.h into drm-internels.rst (Daniel)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190112193251.20450-2-sam@ravnborg.org