drm/xe/vram: start tracking the io_size

First step towards supporting small-bar is to track the io_size for
vram. We can longer assume that the io_size == vram size. This way we
know how much is CPU accessible via the BAR, and how much is not.
Effectively giving us a two tiered vram, where in some later patches we
can support different allocation strategies depending on if the memory
needs to be CPU accessible or not.

Note as this stage we still clamp the vram size to the usable vram size.
Only in the final patch do we turn this on for real, and allow distinct
io_size and vram_size.

v2: (Lucas):
  - Improve the commit message, plus improve the kernel-doc for the
    io_size to give a better sense of what it actually is.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>