drm/amdkfd: Use pr_debug to print the message of reaching event limit

People are inclined to think of the previous pr_warn message as an
error, so use pre_debug instead.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>