drm/amdkfd: Add format attribute to kfd_smi_event_add

To enable compiler type-checked against the format string in callers.

All warnings (new ones prefixed by >>):

>> warning: function 'kfd_smi_event_add' might be a candidate for
'gnu_printf' format attribute [-Wsuggest-attribute=format]

Fixes: d58b8a99 ("drm/amdkfd: Add SMI add event helper")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>