Document and add support for solid_fill property to drm_plane. In
addition, add support for setting and getting the values for solid_fill.

To enable solid fill planes, userspace must assign a property blob to
the "solid_fill" plane property containing the following information:

struct drm_mode_solid_fill {
	u32 r, g, b, pad;
};
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Sebastian Wick <sebastian@sebastianwick.net>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231027-solid-fill-v7-2-780188bfa7b2@quicinc.com