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 <[email protected]>
Cc: Dale B Stimson <[email protected]>
Cc: Michał Winiarski <[email protected]>
Reviewed-by: Michał Winiarski <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: ickle