swr: don't export swr_create_screen_internal

With earlier rework the user and provider of the symbol are within the
same binary. Thus there's no point in exporting the function.

Spotted while reviewing patch from Chuck, that nearly added another
unneeded PUBLIC function.

Cc: Chuck Atkins <chuck.atkins@kitware.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Fixes: f50aa214 "(swr: build driver proper separate from rasterizer")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Chuck Atkins <chuck.atkins@kitware.com>
Reviewed-By: George Kyriazis <george.kyriazis@intel.com<mailto:george.kyriazis@intel.com>>
Tested-by: Chuck Atkins <chuck.atkins@kitware.com<mailto:chuck.atkins@kitware.com>>
(cherry picked from commit 54d84489)