gbm/dri: Fix sign-extension in modifier query

gbm/dri: Fix sign-extension in modifier query

When we were assembling the unsigned 64-bit query return from its
two signed 32-bit component parts, the lower half was getting
sign-extended into the top half. Be more explicit about what we want to
do.

Fixes gbm_bo_get_modifier() returning ((1 << 64) - 1) rather than
((1 << 56) - 1), i.e. DRM_FORMAT_MOD_INVALID.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>