if downscaling is enabled plane data rate increases according to scaling
amount. take scaling amount under consideration while calculating plane
data rate

v2: Address Matt's comments, where data rate was overridden because of
missing else.

v3 (by Matt):
 - Add braces to 'else' branch to match kernel coding style
 - Adjust final calculation now that skl_plane_downscale_amount()
   returns 16.16 fixed point value instead of a decimal fixed point

v4 (by Matt):
 - Avoid integer overflow by making sure final multiplication is
   treated as 64-bit.

Cc: matthew.d.roper@intel.com
Signed-off-by: Kumar, Mahesh <mahesh1.kumar@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Kumar Mahesh <mahesh1.kumar@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463695381-21368-1-git-send-email-matthew.d.roper@intel.com