drm/i915: Use intel_crtc_state in sanitize_watermarks() too

Get rid of all instances of drm_crtc_state, and rename cstate to
crtc_state for more clarity.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628085517.31886-5-maarten.lankhorst@linux.intel.com