drm/i915: work around false-positive maybe-uninitialized warning

gcc-9 gets confused by the code flow in check_dirty_whitelist:

drivers/gpu/drm/i915/gt/selftest_workarounds.c: In function 'check_dirty_whitelist':
drivers/gpu/drm/i915/gt/selftest_workarounds.c:492:17: error: 'rsvd' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I could not figure out a good way to do this in a way that gcc
understands better, so initialize the variable to zero, as last
resort.

Fixes: aee20aaed887 ("drm/i915: Implement read-only support in whitelist selftest")
Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit cc649a9eafc1ef5c40db023084cb94422d08aa84)
Signed-off-by: Joonas Lahtinen <[e-mail]>

drivers/gpu/drm/i915/gt/selftest_workarounds.c

···
 				err = -EINVAL;
 				goto out_unpin;
 			}
+		} else {
+			rsvd = 0;
 		}
 
 		expect = results[0];