drm/amd/display: Remove superfluous assert

[Why]
For loop below the assert already checks for the number of instances to
create. ASSERT is meaningless and causing spam.

[How]
dd

Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>