backends/drm: make finding connector<->crtc combinations more efficient

Encoders are not really relevant for the test result, except that one of
the encoders for the connector must be compatible with the crtc.
The kernel usually exposes only a single encoder per connector for this
reason, but if a driver exposes multiple then that means KWin will do a
lot more tests than is necessary.
In order to prevent that from happening, do fewer syscalls and simplify
code, only check supported encoders once per connector.