drm/xe: Promote VRAM initialization function to own file

There is no point in mixing register access and VRAM code in the
same file. Move and rename the VRAM probe function to a new file
(there are no other changes other then new simple kernel-doc).

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240530133527.1328-5-michal.wajdeczko@intel.com