drm/amd/display: Drive-by fixes for display_mode_vba

Fixes for the following:
- Incorrect pointer type (unsigned int instead of double)
- Incorrect DSC number of slices setting

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>