drm/i915: Rename 'do_execbuf' to 'execbuf_submit'

The submission portion of the execbuffer code path was abstracted into a
function pointer indirection as part of the legacy vs execlist work. The two
implementation functions are called 'i915_gem_ringbuffer_submission' and
'intel_execlists_submission' but the pointer was called 'do_execbuf'. There is
already a 'i915_gem_do_execbuffer' function (which is what calls the pointer
indirection). The name of the pointer is therefore considered to be backwards
and should be changed.

This patch renames it to 'execbuf_submit' which is hopefully a bit clearer.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>