dri3: Always flush glamor before sharing pixmap storage with clients

Even if glamor_gbm_bo_from_pixmap / glamor_fd_from_pixmap themselves
don't trigger any drawing, there could already be unflushed drawing to
the pixmap whose storage we share with a client.

(Ported from amdgpu commit 4b17533fcb30842caf0035ba593b7d986520cc85)
Acked-by: Alex Deucher <alexander.deucher@amd.com>