drm: Add solid fill pixel source

Add "SOLID_FILL" as a valid pixel source. If the pixel_source property is
set to "SOLID_FILL", it will display data from the drm_plane "solid_fill"
blob property.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Sebastian Wick <sebastian@sebastianwick.net>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231027-solid-fill-v7-3-780188bfa7b2@quicinc.com