drm/xe: Change return type of detect_bar2_dgfx() from s64 to u64

drm/xe: Change return type of detect_bar2_dgfx() from s64 to u64

The function never returns a negative value, and the return value is
assigned to a u64 variable. Use u64 for better type correctness
and clarity.

v2: add assert to catch theoretical negative or zero stolen size. (Rodrigo)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://lore.kernel.org/r/20251009230239.2830207-8-shuicheng.lin@intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>