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: aee20aa ("drm/i915: Implement read-only support in whitelist selftest")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200527140526.1458215-2-arnd@arndb.de

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

@@ -623,6 +623,8 @@ static int check_dirty_whitelist(struct intel_context *ce)
 				err = -EINVAL;
 				goto out_unpin;
 			}
+		} else {
+			rsvd = 0;
 		}
 
 		expect = results[0];