drm: Make drm_clflush_virt_range() void*

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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>