Simplify drmmode_handle_transform

Set crtc->driverIsPerformingTransform for any case we can handle before
calling xf86CrtcRotate. We already clear it afterwards when the latter
clears crtc->transform_in_use.

This should allow our separate scanout buffer mechanism to be used in
more cases.

(Cherry picked from amdgpu commit 8e544b4a0de6717feb4abf00052d57c5b726b5ce)
Acked-by: Alex Deucher <alexander.deucher@amd.com>