Revert "drm/amdkfd: flush TLB after updating GPU page table"

This reverts commit 1704ac8e.

After "drm/amdgpu: flush TLB if valid PDE turns into PTE" is checked
in, this workaround is not needed.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>