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

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: 2d3b2b6655d8 ("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
(cherry picked from commit d04a661a2c7169b48782aa5e9d85d4b4383d562e)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>