drm/amd/display: Prevent overflow on DTO calculation

[Why]
uint32_t is implicitly converted to uint64_t while multiplication
still happens on uint32_t side.  This creates digit overflow
for large pixel clock which is meant to be retained in uint64_t.

[How]
Calculate multiplication of units in uint64_t domain instead of
uint32_t in DTO parameter clock caculation.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Chris Park <chris.park@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>