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