drm/amd/display: Initialize stack variable

[WHY]
The stack variable "val" is potentially unpopulate it, so initialize it
with the value 0xf (indicating an invalid mux)

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>