MLK-16953: drm: imx: dcss: Add propriety to change global alpha priority

MLK-16953: drm: imx: dcss: Add propriety to change global alpha priority

This patch adds 'use_global_alpha' property to the primary plane, so that
one can choose whether to use global alpha instead of per-pixel alpha,
when the framebuffer has per-pixel alpha.

Framebuffers that do not have per-pixel alpha will always use global
alpha.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>