drm/amd/display: Don't check for NULL divisor in fixpt code

[Why]
We check for a NULL divisor but don't act on it.
This check does nothing other than throw a warning.
It does confuse static checkers though:
See https://lkml.org/lkml/2025/4/26/371



[How]
Drop the ASSERTs in both DC and SPL variants.

Fixes: 4562236b ("drm/amd/dc: Add dc display driver (v2)")
Fixes: 6efc0ab3 ("drm/amd/display: add back quality EASF and ISHARP and dc dependency changes")
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>