drm/i915/gem: Flush coherency domains on first set-domain-ioctl

Avoid skipping what appears to be a no-op set-domain-ioctl if the cache
coherency state is inconsistent with our target domain. This also has
the utility of using the population of the pages to validate the backing
store.

The danger in skipping the first set-domain is leaving the cache
inconsistent and submitting stale data, or worse leaving the clean data
in the cache and not flushing it to the GPU. The impact should be small
as it requires a no-op set-domain as the very first ioctl in a
particular sequence not found in typical userspace.

Reported-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Fixes: 754a2544 ("drm/i915: Skip object locking around a no-op set-domain ioctl")
Testcase: igt/gem_mmap_offset/blt-coherency
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Cc: <stable@vger.kernel.org> # v5.2+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201019203825.10966-1-chris@chris-wilson.co.uk