drm/amd/display: add a func to disable accelerated mode

[Why]
When driver disabled, we driver force the YCbCr420 to RGB,
which means some register will be changed, such as
RDPCS_PHY_DP_MPLLB_TX_CLK_DIV changed from 1 to 0
When driver re-enabled, OS will Set Mode YCbCr420 again,
which means the register RDPCS_PHY_DP_MPLLB_TX_CLK_DIV
should to be 1 again, but dmub fw can’t update the
register to 1 due to the mpll is not off

[How]
Adds an interface to disable accelerated mode bit,
which allows DM to decide to call during driver
disable/unload scenarios.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Yao Wang1 <Yao.Wang1@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>