drm/komeda: Clean warnings: candidate for 'gnu_printf’ format attribute

komeda/komeda_event.c: In function ‘komeda_sprintf’:
komeda/komeda_event.c:31:2: warning: function ‘komeda_sprintf’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
  num = vsnprintf(str->str + str->len, free_sz, fmt, args);

v2: Update the comment msg.

Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114100421.30510-1-james.qian.wang@arm.com