drm/i915/selftests: Check that whitelisted registers are accessible

There is no point in whitelisting a register that the user then cannot
write to, so check the register exists before merging such patches.

v2: Mark SLICE_COMMON_ECO_CHICKEN1 [731c] as write-only
v3: Use different variables for different meanings!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dale B Stimson <dale.b.stimson@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301140404.26690-6-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20190301160108.19039-1-chris@chris-wilson.co.uk