drm/amd/display: Fill prescale_params->scale for RGB565

[Why]
An assertion is thrown when using SURFACE_PIXEL_FORMAT_GRPH_RGB565
formats on DCE since the prescale_params->scale wasn't being filled.

Found by a dmesg-fail when running the
igt@kms_plane@pixel-format-pipe-a-planes test on Baffin.

[How]
Fill in the scale parameter.
Signed-off-by: NNicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c

@@ -242,6 +242,9 @@ static void build_prescale_params(struct ipp_prescale_params *prescale_params,
 	prescale_params->mode = IPP_PRESCALE_MODE_FIXED_UNSIGNED;
 
 	switch (plane_state->format) {
+	case SURFACE_PIXEL_FORMAT_GRPH_RGB565:
+		prescale_params->scale = 0x2082;
+		break;
 	case SURFACE_PIXEL_FORMAT_GRPH_ARGB8888:
 	case SURFACE_PIXEL_FORMAT_GRPH_ABGR8888:
 		prescale_params->scale = 0x2020;