drm/amd/display: On clock init, maintain DISPCLK freq

[Why]
On init if a display is connected, we need to maintain the DISPCLK
frequency Even though DPG_EN=1, the display still requires the correct
timing or it could cause audio corruption (if DISPCLK freq is reduced).

[How]
Read the current DISPCLK freq and request the same value to ensure the
timing is valid and unchanged.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Chris Park <chris.park@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>