drm/amd/display: augment display clock in dc_cap structure

[Why]
Allow dc report maximum display clock possible at vmin

Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Max Tseng <max.tseng@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>