dri2: Re-use previous CRTC when possible if pick_best_crtc returns NULL

This way, the MSC will continue ticking at the rate of (the last mode
which was enabled for) that CRTC, instead of the client running
unthrottled.

(Ported from amdgpu commit 3109f088fdbd89c2ee8078625d4f073852492656)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>