drm/i915: Implement read-only support in whitelist selftest

Newer hardware supports extra feature in the whitelist registers. This
patch updates the selftest to test that entries marked as read only
are actually read only.

v2: Removed all use of 'rsvd' for read-only registers to avoid
ambiguous code or error messages.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
CC: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712070745.35239-3-John.C.Harrison@Intel.com