drm/amd/display: set av_mute in hw_init for HDMI

[Description]
OS will reserve HW state in UEFI mode.
Driver init_hw reset to RGB which caused HDMI green in YCbCr mode.
read HW blank_color based on acc_mode.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>