egl/dri2: use drm macros to construct device name

egl/dri2: use drm macros to construct device name

Don't hardcode /dev/dri/card0 but instead use the drm
macros which allows the correct /dev/drm0 device to be
opened on OpenBSD.

v2: use snprintf and fallback to /dev/dri/card0
v3: check for snprintf truncation

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>