xf86drm: adjust device node path for minor base

When constructing a path to a device node the minor number retrieved
from fstat needs to have the offset of the node type subtracted from it.
Control and render node types have the same major as the primary node
but each has their own block of minor types at fixed offsets.

v2: remove min < base test as requested by Emil

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>