drm/i915: setup the LMEM region

Hook up the LMEM region. Addresses will start from zero, and for CPU
access we get LMEM_BAR which is just a 1:1 mapping of said region.

Based on a patch from Michel Thierry.

v2 by Jani:
- use intel_uncore_read/intel_uncore_write
- remove trailing blank line

v3: s/drm_info/drm_dbg for info which in non-pertinent for the user

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210127131417.393872-2-matthew.auld@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>