drm/i915/drm: Fix comment for YCbCr20 qp table declaration

Fix comment for YCbCr420 qp table declaration of max value
where the min value is 4 and the max value is 12/15/18
depending on bpc.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230705051502.2568245-3-suraj.kandpal@intel.com