package/xdriver_xf86-video-ati: only select libdrm if the DRI driver is enabled

The xf86 radeon driver does not need libdrm, unless DRI is enabled too.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>