dma-buf: Include missing parentheses to documentation

The missing parentheses prevents the function to be rendered as a link
in the documentation. So, add the missing parentheses to the function on
the documentation.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223121909.149980-1-mcanal@igalia.com