drm/mxsfb: Use gem_free_object_unlocked

CMA helpers are struct_mutex free, and so is the mxsfb itself. And
that's the only valid reason for using gem_free_object.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20170717151045.4188-1-daniel.vetter@ffwll.ch