drm/i915/icl: 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.

Based on the CNL patch by Nanley Chery.

v2: Rebase.

Cc: Anuj Phogat <anuj.phogat@intel.com>
Cc: Nanley Chery <nanley.g.chery@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Requested-by: Anuj Phogat <anuj.phogat@intel.com>
Tested-by: Anuj Phogat <anuj.phogat@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130134918.32283-10-paulo.r.zanoni@intel.com