backends/native: Split hw supported CRTC rotation modes

Those will need a separate treatment from the modes that we eventually
support through "software", so split those into a separate enum so we
can can do the right thing when applying the configuration.

Also, add a helper function that returns the transform that the software
fallbacks should perform, which should be "normal" if the rotation is
already handled via hw.

The function applying the configuration has been modified to always set
a HW rotation mode (even if normal), when we come to support SW rotation
modes, we'll be relying on a normal transformation, so it will be
necessary to have mixed HW/SW managed transforms.

https://bugzilla.gnome.org/show_bug.cgi?id=745079