drm/i915/guc: Fix flag query helper function to not modify state

A flag query helper was actually writing to the flags word rather than
just reading. Fix that. Also update the function's comment as it was
out of date.

NB: No need for a 'Fixes' tag. The test was only ever used inside a
BUG_ON during context registration. Rather than asserting that the
condition was true, it was making the condition true. So, in theory,
there was no consequence because we should never have hit a BUG_ON
anyway. Which means the write should always have been a no-op.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217212942.629922-1-John.C.Harrison@Intel.com