drm/arm/malidp: Disable pixel alpha blending for colors that do not have alpha

Mali dp needs to disable pixel alpha blending (use layer alpha blending) to
display color formats that do not contain alpha bits per pixel

This patch depends on:

"[PATCH v2 01/19] drm/fourcc: Add a alpha field to drm_format_info"

Signed-off-by: Ayan Kumar Halder <ayan.halder@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>