Currently drm_cflush_virt_rage() takes a char* so the caller probably
has to do pointless casting to avoid compiler warnings. Make the
argument void* instead to avoid such issues.

v2: Use void* arithmetic (Chris)
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Acked-by: NDave Airlie <airlied@redhat.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>