Allow/Fix use of multiple ZaphodHead outputs per x-screen. (v2)

Defining multiple ZaphodHead outputs per x-screen in a
multiple x-screen's per gpu configuration caused all
outputs except one per x-screen to go dark, because
there was a fixed mapping x-screen number -> crtc number,
limiting the number of crtc's per x-screen to one.

On a ZaphodHead's setup, be more clever and assign
as many crtc's to a given x-screen as there are
ZaphodHeads defined for that screen, assuming
there are enough unused crtc's available.

Tested on a triple display setup with different combos
of one, two or three ZaphodHeads per one, two or three
x-screens.

This is a port of similar code from xf86-video-nouveau.

v2: Implement suggestions by Michel Dänzer: Less verbose
    debug output, more clear warning message on crtc allocation
    failure. Move clearing of per gpu assigned_crtc mask to
    CloseScreen, indeed testing shows no need for the more
    complex new server generation check from v1.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
(v1) Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

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