gallium/winsys/kms: Fix dumb buffer bpp

The bpp in the dumb buffer creation request is hardcoded to 32, which is an
incorrect assumption as the caller is free to pick any pipe format. Use the
bpp supplied to us through util_format_get_blocksizebits().

Fixes: 3b176c44 "gallium: Add a dumb drm/kms winsys backed swrast provider"
Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit ec0a68e5)