drm: Update drm_cache.c to pull in the new drm_cache.h

The prototypes were moved to a new header, but the function definitions
were not updated to pull in the declarations.

drivers/gpu/drm/drm_cache.c:79:1: warning: no previous prototype for ‘drm_clflush_pages’ [-Wmissing-prototypes]
drivers/gpu/drm/drm_cache.c:120:1: warning: no previous prototype for ‘drm_clflush_sg’ [-Wmissing-prototypes]
drivers/gpu/drm/drm_cache.c:152:1: warning: no previous prototype for ‘drm_clflush_virt_range’ [-Wmissing-prototypes]

Fixes: f9a87bd7 ("drm: Move drm_clflush prototypes to drm_cache header file")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170121181944.24672-1-chris@chris-wilson.co.uk