drm: Add getfb2 ioctl

getfb2 allows us to pass multiple planes and modifiers, just like addfb2
over addfb.

Changes since v2:
 - add privilege checks from getfb1 since handles should only be
   returned to master/root

Changes since v1:
 - unused modifiers set to 0 instead of DRM_FORMAT_MOD_INVALID
 - update ioctl number

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Juston Li <juston.li@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217034642.3814-1-juston.li@intel.com