drm/amd/display: remove variable active_disp

The variable active_disp is being initialized with a value that
is never read, it is being re-assigned immediately afterwards.
Clean up the code by removing the need for variable active_disp.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>