drm/i915: Update workarounds selftest for read only regs

Updates the live_workarounds selftest to handle whitelisted
registers that are flagged as read only.

Signed-off-by: Robert M. Fosha <robert.m.fosha@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@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/20190618010108.27499-5-John.C.Harrison@Intel.com