drm: Move drm_encoder_crtc_ok() to core

Using the new drm_crtc_mask() function, drm_encoder_crtc_ok() can now be
written in a significantly shorter way, so it can be moved to a header
file and be made static inline.

Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --cc drivers/gpu/drm/drm_crtc_helper.c
Simple merge

diff --cc include/drm/drm_crtc.h
Simple merge