backends/drm: fix testing for more connectors than CRTCs

Xaver Huglrequested to merge
work/zamundaaa/cherry-pick into Plasma/6.3

The connectors may not actually be enabled, so the checks could sometimes result in wrongly returning Error::NotEnoughCrtcs.

BUG: 500819

CCBUG: 500797

(cherry picked from commit aa11d89a)

Edited by Xaver Hugl

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