libdrm: introduce drmModeGetConnectorPossibleCrtcs

Nowadays, users don't really care about encoders except for retrieving
the list of CRTCs compatible with a connector. Introduce a new function
so that users no longer need to deal with encoders.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
6 jobs for drm-mode-get-connector-possible-crtcs in 31 seconds (queued for 3 seconds)
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passed #6136706
debian

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passed #6136709
meson-aarch64

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passed #6136710
meson-armhf

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passed #6136708
meson-i386

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meson-ppc64el

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meson-x86_64

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