drm: qxl: Open code probing sequence for qxl

This avoids using the deprecated drm_get_pci_dev() and load() hook
interfaces in the qxl driver.

The only tricky part is to ensure TTM debugfs initialization happens
after the debugfs root node is created, which is done by moving that
code into the debufs_init() hook.

Tested on qemu with igt and running a WM on top of X.

Changes since v1:
 - Drop verification for primary minor in qxl_debugsfs_init.
Changes since V2:
 - Put new header together with other debugfs headers.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119134806.8926-1-krisman@collabora.co.uk