drm: mali-dp: add malidp_crtc_state struct

Add a custom CRTC state struct to enable storing driver's private
state. This patch only adds the base drm_crtc_state struct and
the atomic functions that handle it.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>