drm/i915: Remove allow_alloc from i915_gem_object_get_sg*

This reverts the rest of 0edbb9ba

 ("drm/i915: Move cmd parser
pinning to execbuffer").  Now that the only user of i915_gem_object_get_sg
without allow_alloc has been removed, we can drop the parameter.  This
portion of the revert was broken into its own patch to aid review.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714193419.1459723-4-jason@jlekstrand.net