drm/i915: Do not spam log with missing arch support

Following what was done in drm_cache.c, when the stub for
remap_io_mapping() was added in commit 67c430bb ("drm/i915: Skip
remap_io_mapping() for non-x86 platforms"), it included a log message
with pr_err().  However just the warning is already enough and switching
to WARN_ONCE() allows us to keep the log message while avoiding log
spam.
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220131165926.3230642-4-lucas.demarchi@intel.com

drivers/gpu/drm/i915/i915_mm.h

@@ -22,8 +22,7 @@ int remap_io_mapping(struct vm_area_struct *vma,
 		     unsigned long addr, unsigned long pfn, unsigned long size,
 		     struct io_mapping *iomap)
 {
-	pr_err("Architecture has no %s() and shouldn't be calling this function\n", __func__);
-	WARN_ON_ONCE(1);
+	WARN_ONCE(1, "Architecture has no drm_cache.c support\n");
 	return 0;
 }
 #endif