xf86drm: Allocate drmDevicePtr's on stack

xf86drm: Allocate drmDevicePtr's on stack

Currently we dynamically allocate 16 pointers and reallocate more as
needed.

Instead, allocate the maximum number (256) on stack - the number is
small enough and is unlikely to change in the foreseeable future.

This allows us to simplify the error handling and even shed a few bytes
off the final binary.

v2:
 - add a define & description behind the magic 256
 - report error to strerr and skip when over 256 device nodes

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com> (v1)
Reviewed-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v1)