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

This reverts commit b00fb976.

Reason for revert: this causes some tests fail with call trace.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Acked-by: Yang Wang <kevinyang.wang@amd.com>