drm/vmwgfx: Fix a layout race condition

This fixes a layout update race condition. We make sure
the crtc mutex is locked before we dereference crtc->state. Otherwise the
state might change under us.

Since now we're already holding the crtc mutexes when reading the gui
coordinates, protect them with the crtc mutexes rather than with the
requested_layout mutex.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>