Properly clip the source when the destination gets clipped
by the pipe dimensions.

Sadly the video sprite hardware is rather limited so it can't do proper
sub-pixel postitioning. Resort to truncating the source coordinates to
(macro)pixel boundary.

The scaling checks are done using the strict drm_region functions.
Which means that an error is returned when the min/max scaling
ratios are exceeded.

Also do some additional checking against various hardware limits.

v2: Truncate src coords instead of rounding to avoid increasing src
    viewport size, and adapt to changes in drm_calc_{h,v}scale().
v3: Adapt to drm_region->drm_rect rename. Fix misaligned crtc_w for
    packed YUV formats when scaling isn't supported.
v4: Use stricter scaling checks, use drm_rect_equals()
v5: If sprite is below min size, make it invisible instead returning
    an error.
    Use WARN_ON() instead if BUG_ON(), and add one to sanity check the
    src viewport size.
v6: Add comments to remind about src and dst coordinate types
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>