drm/amd/display: Fix compiler warnings on high compiler warning levels

drm/amd/display: Fix compiler warnings on high compiler warning levels

[why]
Enabling higher compiler warning levels results in many issues that can
be trivially resolved as well as some potentially critical issues.

[how]
Fix all compiler warnings found with various compilers and higher
warning levels.  Primarily, potentially uninitialized variables and
unreachable code.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>