drm/i915: Add test for invalid flag bits in whitelist entries

As per review feedback by Tvrtko, added a check that no invalid bits
are being set in the whitelist flags fields.

Also updated the read/write access definitions to make it clearer that
they are an enum field not a set of single bit flags.

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/20190712070745.35239-2-John.C.Harrison@Intel.com