drm/amd/display: Add debug option to override DSC target bpp increment

[why]
It's required for debug purposes.

[how]
Add a dsc_bpp_increment_div debug option that overrides DPCD
BITS_PER_PIXEL_INCREMENT value. The value dsc_bpp_increment_div should
be set to is the one after parsing, i.e. it could be 1, 2, 4, 8 or 16
(meaning 1pix, 1/2pix, ..., 1/16pix).

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>