pvr: Assorted error handling fixes

The changes here are mostly centred around allowing correct VkResult errors to propogate up from the lower levels of the driver.

Other changes include:

• Use of the common physical device enumeration framework,
• Lifting of some common handling out of the winsys implementations,
• Pushing some handling down into the winsys layer itself (not the implementations), and
• Renaming of fds to make their use cases more obvious.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>