swr: Remove unnecessary memset call

Zeroing memory after calloc is not necessary. This also allows to avoid
possible crash when allocation fails, because memset is called before
checking screen for NULL.

Fixes: a29d63ec "swr: refactor swr_create_screen to allow
                              for proper cleanup on error"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 9d3a2394)