backends/native: Add fake cursor plane using impl_device's crtcs

Because meta_kms_impl_device_simple_initable_init is called in the middle of meta_kms_device_new, the crtcs list for MetaKmsDevice has not been populated yet. And thus the loop to detect missing cursor planes and create fake ones never iterated. But the crtcs list does already exist in MetaKmsImplDevice so iterate over that instead.

Closes: #3264