drm: Convert cmpxchg(bool) back to a two step operation

ARM v6 (at least) only allows cmpxchg on 32bit variables which doesn't
always include the bool type.

drivers/built-in.o: In function `vblank_disable_and_save':
imx-ocotp.c:(.text+0xb45e8): undefined reference to `__bad_cmpxchg'
Makefile:986: recipe for target 'vmlinux' failed

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Fixes: 43dc7fe2

 ("drm: Mark up accesses of vblank->enabled outside of its spinlock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330140832.32377-1-chris@chris-wilson.co.uk