modesetting: Fix msSharePixmapBacking Segfault Regression
Commit cb1b1e18 modified msSharePixmapBacking() to derive modesettingPtr from the 'screen' argument. Unfortunately, the name of the argument is misleading -- the screen is the slave screen. If the master is modesetting, and the slave is not modesetting, it will segfault.
To fix the problem, this change derives modesettingPtr from ppix->drawable.pScreen. This method is already used when calling ms->glamor.shareable_fd_from_pixmap() later in the function.
To avoid future issues, this change also renames the 'screen' argument to 'slave'.
Signed-off-by: Alex Goins agoins@nvidia.com