drm/i915: Add MIPI mmio reg base

This patch adds a mmio base address variable for DSI display,
to make the DSI code generic, so that, if required, the same code
can be re-used for future platforms with different mmio base.

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Appease checkpatch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>