drm/amd/display: Fix blend LUT programming

[Why]
LUT write index does not get reset to zero when writing the LUT values
for each separate RGB component, which results in wrong data for 2 of
the 3 components.

[How]
Reset LUT write index to zero before writing each component's data.

Reviewed-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>