lib/xe/xe_gt: Repair xe_gt_stats_get_count function

Resolve the following issues:

- The target directory for the fopen does not take into consideration
  the dri subdirectory and assumes the correct path is dri/0.  This
  might not be the case.  Use the "minor" function and fstat to
  generate the correct subdirectory.

- The fopen may fail and return a NULL pointer, causing the subsequent
  fgets to generate a SIGSEGV.  Assert the file opened correctly.

- The file opened by fopen is never closed.  Close it.

v2: fclose (Kamil)

Fixes: 10cefd82 ("tests/intel/xe_tlb: Add test to check TLB invalidation")
Closes: drm/xe/kernel#3212


Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Suggested-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
CC: Sai Gowtham Ch <sai.gowtham.ch@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>