drm/i915/cnl: Allow the reg_read ioctl to read the RCS TIMESTAMP register

This enables the Mesa driver to advertise support for ARB_timer_query, and
thus an OpenGL version higher than 3.2.

Suggested-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170905184507.30046-1-nanley.g.chery@intel.com