drm/ast: Actually load DP501 firmware when required

The ast driver has a code to load the DP501 firmware, but it's never
used.  This patch implements its actual usage by requesting the
firmware on demand, and release the firmware at exit as well.

Also the path contains a few cleanups and makes relevant functions
static.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>