drm/xe: Add xe_mmio_init() initialization function

drm/xe: Add xe_mmio_init() initialization function

Add a convenience function for minimal initialization of struct xe_mmio.
This function also validates that the entirety of the provided mmio region
is usable with struct xe_reg.

v2: Modify commit message, add kernel doc, refactor assert (Michal)
v3: Fix off-by-one bug, add clarifying macro (Michal)
v4: Derive bitfield width from size (Michal)

Signed-off-by: Ilia Levi <ilia.levi@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250213093559.204652-1-ilia.levi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>