drm/i915: Fix format string truncation warning

drm/i915: Fix format string truncation warning

GCC notices that the 16-byte uabi_name field could theoretically be too
small for the formatted string if the instance number exceeds 100.

So grow the field to 20 bytes.

drivers/gpu/drm/i915/intel_memory_region.c: In function ‘intel_memory_region_create’:
drivers/gpu/drm/i915/intel_memory_region.c:273:61: error: ‘%u’ directive output may be truncated writing between 1 and 5 bytes into a region of size between 3 and 11 [-Werror=format-truncation=]
  273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
      |                                                             ^~
drivers/gpu/drm/i915/intel_memory_region.c:273:58: note: directive argument in the range [0, 65535]
  273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
      |                                                          ^~~~~~
drivers/gpu/drm/i915/intel_memory_region.c:273:9: note: ‘snprintf’ output between 7 and 19 bytes into a destination of size 16
  273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  274 |                  intel_memory_type_str(type), instance);
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 3b38d3515753 ("drm/i915: Add stable memory region names")
Cc: <stable@vger.kernel.org> # v6.8+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20251205113500.684286-2-ardb@kernel.org
(cherry picked from commit 18476087f1a18dc279d200d934ad94fba1fb51d5)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>