drm/i915/gem: Ignore readonly failures when updating relocs

drm/i915/gem: Ignore readonly failures when updating relocs

If the user passes in a readonly reloc[], by the time we notice we have
already committed to modifying the execobjects, or have indeed done so
already. Reporting the failure just compounds the issue as we have no
second pass to fall back to anymore.

"Be damned if you do, and damned if you don't."

Testcase: igt/gem_exec_reloc/readonly
Fixes: dd8c1a4bb35e ("drm/i915/gem: Drop relocation slowpath")
References: ae5ce2045639 ("drm/i915: Force the slow path after a user-write error")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331162150.3635-1-chris@chris-wilson.co.uk
(cherry picked from commit 97a37c919f6262fe75afc4a4eb838093bf18b032)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>