drm/fb-helper: Do the 'max_conn_count' zero check

Since we cannot make sure the 'max_conn_count' will always be none
zero from the users, and then if max_conn_count equals to zero, the
kcalloc() will return ZERO_SIZE_PTR, which equals to ((void *)16).

So this patch fix this with just doing the 'max_conn_count' zero check
in the front of drm_fb_helper_init().

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
CC: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>