drm/cache: Use wbinvd helpers

When the original drm code was written there were no centralized functions for
doing a coordinated wbinvd across all CPUs. Now (since 2010) there are, so use
them instead of rolling a new one.

v2: On x86 UP systems the wbinvd_on_all_cpus() is defined as a static inline in
smp.h. We must therefore include this file so we don't get compiler errors.
This error was found by 0-DAY kernel test infrastructure. We only need this for
x86.

Cc: Intel GFX <intel-gfx@lists.freedesktop.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>