drm/i915/reg: use the correct register to access SAGV block time

Wrong register address is used to read the SAG block time. Fix
the register address according to the bspec.

Bspec: 64608

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323114426.41136-3-vinod.govindapillai@intel.com