drm/amd/display: Configure DTBCLK_P with OPTC only for dcn401

[WHY]
DTBCLK_P is used to generate virtual pixel clock, and to drive the HPO
stream encoder clock. Programming the required clock when
enabling/disabling both components can cause issues.
For example, if HPO is being disabled and clock source is changed to
REFCLK, virtual pixel rate will then be wrong, causing issues in CRTC.

[HOW]
Only program the DTBCLK_P when programming CRTC, as its expected it will
be enabled prior to HPO, and disabled after HPO in all valid cases.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>