drm: Change {pixel,line,frame}dur_ns from s64 to int

Using s64 for the timestamping constants is wasteful. Signed 32bit
integers get us a range of over +-2 seconds. Presuming that no-one
wants to a vrefresh rate less than 0.5, we can switch to using int
for the timestamping constants. We save a few bytes in drm_crtc and
avoid a bunch of 64bit math.

Reviewed-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>