drm/i915/selftest: Ensure string fits within name[]

Shrink the strncpy bounds to ensure the NUL-terminator can fit within
the embedded array:

In file included from drivers/gpu/drm/i915/gem/i915_gem_context.c:2475:
drivers/gpu/drm/i915/gem/selftests/mock_context.c: In function ‘mock_context’:
drivers/gpu/drm/i915/gem/selftests/mock_context.c:40:3: error: ‘strncpy’ specified bound 24 equals destination size [-Werror=stringop-truncation]
   40 |   strncpy(ctx->name, name, sizeof(ctx->name));
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203181625.589118-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/selftests/mock_context.c

@@ -37,7 +37,7 @@ mock_context(struct drm_i915_private *i915,
 	if (name) {
 		struct i915_ppgtt *ppgtt;
 
-		strncpy(ctx->name, name, sizeof(ctx->name));
+		strncpy(ctx->name, name, sizeof(ctx->name) - 1);
 
 		ppgtt = mock_ppgtt(i915, name);
 		if (!ppgtt)