drm/amdgpu/display: Remove t_srx_delay_us.

Unused. Convert the divisions into asserts on the divisor, to
debug why it is zero. The divide by zero is suspected of causing
kernel panics.

While I have no idea where the zero is coming from I think this
patch is a positive either way.

Cc: stable@vger.kernel.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>