drm/i915/selftests: Report setup errors for igt_partial_tiling

igt_partial_tiling managed to fail with an -EBUSY. This usually means a
pin leak, but that should be impossible given the test setup. Add a
couple of error messages to help identify the path that failed.

References: https://bugs.freedesktop.org/show_bug.cgi?id=105073


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180213120940.21579-1-chris@chris-wilson.co.uk