drm/i915: Read out memory type

We'll need to know the memory type in the system for some
bandwidth limitations and whatnot. Let's read that out on
gen9+.

v2: Rebase
v3: Fix the copy paste fail in the BXT bit definitions (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306203551.24592-13-ville.syrjala@linux.intel.com