drm: drm_fourcc: Add uncompressed AFBC modifier

AFBC has a mode that guarantees use of AFBC with an uncompressed
payloads, we add a new modifier to support this mode.

V2: updated modifier comment

Signed-off-by: Ben Davis <ben.davis@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200430083220.17347-1-ben.davis@arm.com