drm/amd/display: fix integer overflow during MSA V_Freq calculation

[why]
Analyzer shows incorrect V freq in MSA for some large timing.

[how]
Cast an 32 bit integer to uint64_t before multiplication to avoid
integer overflow for a very large timing.

Reviewed-by: Ariel Bernstein <Eric.Bernstein@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>