drm: Use a flexible array member for blob property data

The blob property data is always allocated immediately after the object
header. No need for the extra indirection when accessing it, just use
a flexible array member.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>