drm: Give drm_agp_clear drm_legacy_ prefix

It has a  DRIVER_MODESET check to sure make it's not creating havoc
for drm drivers. Make that clear in the name too.

v2: Move misplaced hunk, spotted by 0day and Thierry.

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-2-git-send-email-daniel.vetter@ffwll.ch