drm/i915: Remove extra multi-gt pm-references

There was an attempt to fix an issue of illegal attempts to free a still
active i915 VMA object when parking a GT believed to be idle, reported by
CI on 2-GT Meteor Lake.  As a solution, an extra wakeref for a Primary GT
was acquired from i915_gem_do_execbuffer() -- see commit f56fe3e9


("drm/i915: Fix a VMA UAF for multi-gt platform").

However, that fix occurred insufficient -- the issue was still reported by
CI.  That wakeref was released on exit from i915_gem_do_execbuffer(), then
potentially before completion of the request and deactivation of its
associated VMAs.  Moreover, CI reports indicated that single-GT platforms
also suffered sporadically from the same race.

Since the issue has now been fixed by a preceding patch "drm/i915/vma: Fix
UAF on destroy against retire race", drop the no longer useful changes
introduced by that insufficient fix.

v3: Also drop the no longer used .wakeref_gt0 field from struct
    i915_execbuffer.
v2: Avoid the word "revert" in commit message (Rodrigo),
  - update commit description reusing relevant chunks dropped from the
    description of the proper fix (Rodrigo).

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240305143747.335367-7-janusz.krzysztofik@linux.intel.com