To enable eventual removal of pr_warning

This makes pr_warn use consistent for drivers/gpu

Prior to this patch, there were 15 uses of pr_warning and
20 uses of pr_warn in drivers/gpu
Acked-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NEdward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>