pvr: Squeeze fd handling into winsys layer

pvr: Squeeze fd handling into winsys layer

This sets us up to make use of the common physical device initialization
code.

As well as lifting the fd handles out of the implementations, this
pushes creation and destruction of the fds into the winsys layer. In
order for this to make sense, the winsys object is now created *before*
each pvr_device or pvr_physical_device. If there's an error setting up
the winsys instance, there's no point in continuing to create either
one.

Also lifts alloc to the winsys layer since there's nothing special about
it in either implementation.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23023>