drm: rcar-du: Refactor DEFR8 feature

Rename the feature from RCAR_DU_FEATURE_DEFR8 to
RCAR_DU_FEATURE_EXT_CTRL_REGS to cover all extended control registers in
addition to the DEFR8 register.

Usage of the feature is refactored to optimize runtime operation and
prepare for external clock support.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>