libdrm: Allow dynamic drm majors on linux

To determine whether a device node is a drm device node or not, the code
currently compares the node's major number to the static drm major device
number.

This breaks the standalone vmwgfx driver on XWayland dri clients,
https://cgit.freedesktop.org/mesa/vmwgfx


and any future attempt to introduce dynamic device numbers for drm.

So instead of checking for the device major, instead check for the presence
of the /sys/dev/char/<major>:<minor>/device/drm directory.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>