drm/amd/display: Clear pipe pointers on pipe reset

[Why]
We want to clean up unnecessary asserts, one of which is an assert in
resource_is_pipe_type that fires if a pipe has no stream and still has
pointers to other pipes ("dangling state"). This gets hit because pipes
are not properly cleaned up in reset_back_end_for_pipe. When resetting a
pipe, the existing MPCC / ODM combine pointers are no longer valid,
especially when we put ODM in bypass.

[How]
 - reset pipe pointers in reset_back_end_for_pipe
 - remove useless code to avoid confusion
     (a long time ago it had a reason to be there, not anymore)

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>