drm: Make property flags u32

The property flags are part of the uabi and we have 32 bits for them.
Pass them around as u32 internally as well, instead of a signed int.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306164849.2862-5-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>