drm: hdlcd: Fix the calculation of the scanout start address

The calculation of the framebuffer's start address was wrongly using
the CRTC's x and y position rather than the one of the source
framebuffer. To fix that we need to update the plane_check code to
call drm_plane_helper_check_state() to clip the src and dst coordinates.
While there so some minor cleanup of redundant freeing of
devm_alloc-ated memory.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>