Fix crash when page flipping in multi-X-Screen/Zaphod mode

amdgpu_do_pageflip() indexed the flipdata->fb[] array
indexing over config->num_crtc, but the flip completion
routines, e.g., drmmode_flip_handler(), index that array
via the crtc hw id from drmmode_get_crtc_id(crtc).

This is mismatched and causes indexing into the wrong
array slot at flip completion -> Server crash.

Always use drmmode_get_crtc_id(crtc) for indexing into
the array to fix this.

Tested on a dual-X-Screen setup with one video output
assigned to each X-Screen, page-flipping an OpenGL app
on either of both X-Screens. This used to crash when
flipping on X-Screen 1, now it doesn't anymore.

Fixes: 9b6782c8 "Store FB for each CRTC in drmmode_flipdata_rec"
(Ported from radeon commit 0058fd2ebf4c900b12f129984e98886a7ac84b2f)
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
12 jobs for zaphod-crash-fix in 4 minutes and 38 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Docker Image
passed #67774
debian-testing

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  Build
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xserver-1.13

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xserver-1.14

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xserver-1.15

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xserver-1.16

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xserver-1.17

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xserver-1.18

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xserver-1.18-clang

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xserver-1.19

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xserver-1.19-clang

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xserver-1.20

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xserver-1.20-clang

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