drm: direct include of drm.h in drm_prime.c

drm: direct include of drm.h in drm_prime.c

Do not rely on including drm.h from drm_file.h,
as the include in drm_file.h will be dropped.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718161507.2047-9-sam@ravnborg.org