drm: Add drm_atomic_set_mode_for_crtc

Add a new helper, to be used later for blob property management, that
sets the mode for a CRTC state, as well as updating the CRTC enable/active
state at the same time.

v2: Do not touch active/mode_changed in CRTC state. Document return
    value. Remove stray drm_atomic_set_mode_prop_for_crtc declaration.

v3: Remove i915 changes, and leave it directly bashing crtc_state->mode
    for the meantime.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>