Add drmModeGetFB2

Add a wrapper around the getfb2 ioctl, which returns extended
framebuffer information mirroring addfb2, including multiple planes and
modifiers.

Changes since v7:
 - add new symbols to core-symbol.txt (Eric Engestrom)

Changes since v5:
 - style change

Changes since v4:
 - Set fb_id at init instead of memclear() and set (Eric Engestrom)

Changes since v3:
 - remove unnecessary null check in drmModeFreeFB2 (Daniel Stone)

Changes since v2:
 - getfb2 ioctl has been merged upstream
 - sync include/drm/drm.h in a seperate patch

Changes since v1:
 - functions should be drm_public
 - modifier should be 64 bits
 - update ioctl number

Signed-off-by: Juston Li <juston.li@intel.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>