drm/i915/gt: Mark up a debug-only function

drivers/gpu/drm/i915//gt/intel_workarounds.c:1394:20: error: function 'is_nonpriv_flags_valid' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static inline bool is_nonpriv_flags_valid(u32 flags)

This is only used by debug build, so mark it as maybe-unused to keep the
compiler from complaining.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210109163455.28466-2-chris@chris-wilson.co.uk