drm: Fix get_property logic fumble

drm: Fix get_property logic fumble

Yet again I've proven that I can't negate conditions :(

Testcase: igt/kms_properties/get_property-sanity
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Fixes: eb8eb02ed850 ("drm: Drop modeset_lock_all from the getproperty ioctl")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170410115445.13829-1-daniel.vetter@ffwll.ch