drm/amd/amdgpu: shorten the gfx idle worker timeout

Shorten the gfx idle worker timeout. This is to sync with
DAL when there is no activity on the screen. Original 1
second can not sync with DAL, so DAL can not apply MALL
when the workload type is not bootup default.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>