drm/i915: Refactor confusing __intel_gt_reset()

__intel_gt_reset() is really for resetting engines though
the name might suggest something else. So add a helper function
to remove confusions with no functional changes.

v2: Move intel_gt_reset_all_engines() next to
    intel_gt_reset_engine() to make diff simple(John)

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422201951.633-1-nirmoy.das@intel.com