drm/i915/gem: Make an alignment check more sensible

What we really want to check is that size of the engines array, i.e.
args->size - sizeof(*user) is divisible by the element size, i.e.
sizeof(*user->engines) because that's what's required for computing the
array length right below the check.  However, we're currently not doing
this and instead doing a compile-time check that sizeof(*user) is
divisible by sizeof(*user->engines) and avoiding the subtraction.  As
far as I can tell, the only reason for the more confusing pair of checks
is to avoid a single subtraction of a constant.

The other thing the BUILD_BUG_ON might be trying to implicitly check is
that offsetof(user->engines) == sizeof(*user) and we don't have any
weird padding throwing us off.  However, that's not the check it's doing
and it's not even a reliable way to do that check.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-21-jason@jlekstrand.net