drm/amd/display: Consider DSC target bpp precision when calculating DSC target bpp

drm/amd/display: Consider DSC target bpp precision when calculating DSC target bpp

[why]
DSC target bpp precision is a decoder DPCD and an AMD encoder capability.
It must be taken into account when calculating target bitrate.

[how]
Add a DC DSC function that does this calculation.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>