drm/i915/tgl: 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 ICL patch by Paulo Zanoni and CNL patch by Nanley Chery.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190726002412.5827-3-lucas.demarchi@intel.com