drm/amd/display: update dsc max_target_bpp to 16 bpp

[why]
According to the latest specs, the max_target bpp sink
can support is 16 bpp.

[how]
update dsc max_target_bpp to 16

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>