drm/amdkfd: Simplify events page allocator

The first event page is always big enough to handle all events.
Handling of multiple events pages is not supported by user mode, and
not necessary.

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>