Instead of relying on a scale which may increase rounding errors,
clip src by doing: src * (dst - clip) / dst and rounding the result
away from 1, so the new coordinates get closer to 1. We won't need
to fix up with a magic macro afterwards, because our scaling factor
will never go to the other side of 1.

Changes since v1:
- Adjust dst immediately, else drm_rect_width/height on dst gives bogus
  results.
Change since v2:
- Get rid of macros and use 64-bits math.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Add Villes comment, and rename newsrc to tmp. (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503112217.37292-3-maarten.lankhorst@linux.intel.com