drm/xe: Carve out wopcm portion from the stolen memory

drm/xe: Carve out wopcm portion from the stolen memory

The top of stolen memory is WOPCM, which shouldn't be accessed. Remove
this portion from the stolen memory region for discrete platforms.
This was already done for integrated, but was missing for discrete
platforms.

This also moves get_wopcm_size() so detect_bar2_dgfx() and
detect_bar2_integrated can use the same function.

v2: Improve commit message and suitable stable version tag(Lucas)

Fixes: d8b52a02cb40 ("drm/xe: Implement stolen memory.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: stable@vger.kernel.org # v6.11+
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250210143654.2076747-1-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>