drm/i915/gem: Drop legacy execbuffer support (v2)

libdrm has supported the newer execbuffer2 ioctl and using it by default
when it exists since libdrm commit b50964027bef which landed Mar 2, 2010.
The i915 and i965 drivers in Mesa at the time both used libdrm and so
did the Intel X11 back-end.  The SNA back-end for X11 has always used
execbuffer2.

v2 (Jason Ekstrand):
 - Add a comment saying what Linux version it's being removed in.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Keith Packard <keithp@keithp.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210317234014.2271006-2-jason@jlekstrand.net