drm: ast,cirrus,mgag200: use drm_can_sleep

these 3 were checking in_interrupt but we have situations where
calling vunmap under this could cause a BUG to be hit in
smp_call_function_many. Use the drm_can_sleep macro instead,
which should stop this path from been taken in this case.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>