drm: add bitmask property type

A bitmask property is similar to an enum.  The enum value is a bit
position (0-63), and valid property values consist of a mask of
zero or more of (1 << enum_val[n]).

[airlied: 1LL -> 1ULL]

Signed-off-by: Rob Clark <rob@ti.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>