drm/amd/display: Properly disable scaling on DCE6

SCL_SCALER_ENABLE can be used to enable/disable the scaler
on DCE6. Program it to 0 when scaling isn't used, 1 when used.
Additionally, clear some other registers when scaling is
disabled and program the SCL_UPDATE register as recommended.

This fixes visible glitches for users whose BIOS sets up a
mode with scaling at boot, which DC was unable to clean up.

Fixes: b70aaf55 ("drm/amd/display: dce_transform: add DCE6 specific macros,functions")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>