drm: Move drm_clflush prototypes to drm_cache header file

Continue to clean up drmP.h by moving the cache flushing functions into
it's own header file.

Compile-tested only

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109215649.6860-2-krisman@collabora.co.uk