drm/amd/display: Remove useless assignments and variables

[WHAT & HOW]
misc0, temp and split_pipe are assigned but immediately re-assigned
to other values. The early assignments are useless and are removed.
Unused variables are removed as well.

This fixes 5 UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>