drm: drm_crtc.h self-contained

While removing drmP.h from drm/radeon a few files ended
up including drm_crtc.h as the first file.
This failed build due to a missing dependency in drm_crtc.h.

Add the missing include file.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
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>
Link: https://patchwork.freedesktop.org/patch/msgid/20190608080241.4958-2-sam@ravnborg.org