drm/i915: move mipi_mmio_base to display.dsi

Move display DSI related members under drm_i915_private display
sub-struct.

Prefer adding anonymous sub-structs even for single members that aren't
our own structs.

Abstract mmio base member access in register definitions in a macro.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dc7c5a871fe558a809ea943eca5c71dfff1740a8.1661779055.git.jani.nikula@intel.com