drm/i915/gt: Rename dev_priv to i915 for private data naming consistency

It has become common practice to refer to the drm_i915_private
structures as "i915". However, there are still instances where
they are referred to as "dev_priv". This inconsistency can make
grepping for information more difficult and does not maintain a
cohesive style throughout the code.

Rename all the "dev_priv" structures in the gt/* directory to
"i915".

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230210150344.1066991-1-andi.shyti@linux.intel.com