drm: Add getfb2 ioctl

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 <[email protected]>
Signed-off-by: Juston Li <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Signed-off-by: Lyude Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]