drm: mali-dp: Don't force source size == crtc size

Remove the check enforcing that src_w and src_h match crtc_w and crtc_h,
as this prevents rotation from working.

The check was intended to disallow scaling, but
drm_plane_helper_check_state() does that for us, while also taking
rotation into account, so the removed check was redundant in any case.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>