drm/amd/display: init dispclk from bootup clock for DCN315

drm/amd/display: init dispclk from bootup clock for DCN315

[Why]
Driver does not pick up and save vbios's clocks during init clocks,
the dispclk in clk_mgr will keep 0.
OS might change the timing (lower the pixel clock) after boot.
Then driver will set the dispclk to lower when safe_to_lower is false,
for in clk_mgr dispclk is zero, it's illegal and causes garbage.

[How]
Dump and save the vbios's clocks, and init the dispclk in
dcn315_init_clocks.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Zhongwei Zhang <Zhongwei.Zhang@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>