drm/i915/dsb: Fix in mmio offset calculation of DSB instance

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: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205123513.22603-1-animesh.manna@intel.com