swr: refactor swr_create_screen to allow for proper cleanup on error

swr: refactor swr_create_screen to allow for proper cleanup on error

This makes the following changes to address cleanup issues:
- Error conditions now return NULL instead of calling exit()
- swr_creen is now freed upon error, rather than leak.
- Library handle from dlopen is now closed upon swr_screen destruction

v2: Added additional context in commit msg and remove unnecessary "PUBLIC"
v3: Fix typo in commit message.

Signed-off-by: Chuck Atkins <chuck.atkins@kitware.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Bruce Cherniak <bruce.cherniak@intel.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
cc: mesa-stable@lists.freedesktop.org