drm/i915/gt: remove i915->gt0 in favour of i915->gt[0]

Since gt0 == i915->gt[0], just drop the former.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5319364f4c9416fed218382d020a57c5b0a17bee.1696236329.git.jani.nikula@intel.com