drm/i915/gt: Make the CTX_TIMESTAMP readable on !rcs

For reasons that be, the HW only allows usersace to read its own
CTX_TIMESTAMP [context local HW runtime] on rcs. Make it available for
all by adding it to the whitelists.

v2: The change took effect from Cometlake.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200602154839.6902-1-chris@chris-wilson.co.uk