drm/amd/display: Check null values from functions

Functions get_per_method_common_meta and get_expanded_strategy_list can
return null and thus it is necessary to check their returned values
before dereferencing.

This fixes 3 NULL_RETURNS issues reported by Coverity.

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