drm/i915/gem: Drop check for changed VM in EXECBUF

Since the introduction of d4433c7600f7 ("drm/i915/gem: Use the proto-context
to handle create parameters (v5)") it has not been possible for VM to change
after context creation so the check will never fail.

Sima's analysis:

This check was added in f7ce8639f6ff ("drm/i915/gem: Split the context's
obj:vma lut into its own mutex") but without any hint in the commit
message as to why. In another hunk of that commit there's a hint though in
__eb_add_lut:

/* user racing with ctx set-vm */

This would mean that this bug was introduced in e0695db7298e ("drm/i915:
Create/destroy VM (ppGTT) for use with contexts"), which allowed to change
the gem_ctx->vm at runtime, opening up the race that was partially fixed
in the earlier referenced commit about a year later.

But it cannot be exploited anymore in anything remotely recent because
with the introduction of proto-contexts we've made gem_ctx->vm invariant
again, exactly to preemptively close all these potential issues.
Specifically d4433c7600f7 ("drm/i915/gem: Use the proto-context to handle
create parameters (v5)") is the vm specific part of the proto-context
work.

v3:
- Include Sima's analysis and WARN_ON_ONCE

v4:
- Focus only on latest mainline codebase

References: https://lore.kernel.org/all/20260324151741.29338-1-sosohero200@gmail.com/
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://patch.msgid.link/20260409053111.8914-1-joonas.lahtinen@linux.intel.com
(cherry picked from commit f6d4afc9ec6a0bc326151b35a7a3369369180079)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>