drm/amdgpu/vcn: fix race condition issue for dpg unpause mode switch

Couldn't only rely on enc fence to decide switching to dpg unpaude mode.
Since a enc thread may not schedule a fence in time during multiple
threads running situation.

v3: 1. Rename enc_submission_cnt to dpg_enc_submission_cnt
    2. Add dpg_enc_submission_cnt check in idle_work_handler

v4:  Remove extra counter check, and reduce counter before idle
    work schedule

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>