As the current usage is restricted to first DSB instance per pipe, so
existing code could not catch the issue to calculate the mmio offset
of different DSB instance per pipe. Corrected the offset calculation.

Fixes: a6e58d9a ("drm/i915/dsb: Check DSB engine status.")
Signed-off-by: NAnimesh Manna <animesh.manna@intel.com>
Reviewed-by: NAnshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205123513.22603-1-animesh.manna@intel.com