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: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>