Set drmmode_crtc->scanout_id = 0 when TearFree is disabled

When disabling TearFree, drmmode_crtc->scanout_id could remain as 1,
but drmmode_set_mode_major would destroy drmmode_crtc->scanout[1], so
scanout_do_update() would keep bailing, and the scanout buffer would
stop being updated.

Fixes freeze after disabling TearFree on a CRTC with active RandR
rotation or other transform.

(Ported from amdgpu commit 7db0c8e9d7586cff4312d4b93684d35de3e6376f)
Acked-by: Alex Deucher <alexander.deucher@amd.com>