drm/amdgpu/dc: Simplify drm_crtc_state::active checks

drm_atomic_crtc_check enforces that ::active can only be true if
::enable is as well.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>