drm/i915: Create MMIO virtual functions

In preparation for having per GEN MMIO functions, create, and start
using MMIO functions in our uncore data structure. This simply makes the
transition easier by allowing us to just plug in the per GEN stuff
later.

For simplicity, I moved the intel_uncore_init() function down since
those rely on static functions defined lower in the file. This is most
of the churn in this patch.

I made one unrelated change here by using off_t datatype for the offset
of the register to write. I like the clarity that this brings to the
code. If I did it as a separate patch, I am pretty certain it would get
bikeshedded to oblivion.

Requested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>