drm/amd/display: Prevent integer overflow when mhz to khz

drm/amd/display: Prevent integer overflow when mhz to khz

[WHAT]
Cast to long long before multiplication to prevent overflow
when converting mhz to khz by multiplying by 1000.

This is reported as INTEGER_OVERFLOW errors by Coverity.

Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>