drm/r600/kms: fixup number of loops per blit calculation.

Some people were seeing
*ERROR* radeon: writting more dword to ring than expected
after certain blits, the loops calculation didn't take
into account that we do a separate blit for the remainder
after doing the aligned blits.

Acked-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>