drm/amd/display: fix 32 bit compilation errors in dc_dmub_srv.c

There are several things wrong here.  First, none of these
numbers are FP, so there is no need to cast to double.  Next
make sure to use proper 64 bit division helpers.

Fixes: 85f4bc0c ("drm/amd/display: Add SubVP required code")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>