drm/i915/reg: fix QGV points register access offsets

Wrong offsets are calculated to read QGV point registers. Fix it
to read from the correct registers.

Bspec: 64602

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323114426.41136-2-vinod.govindapillai@intel.com