drm/ast: Manage release of firmware backup memory

The ast driver keeps a backup copy of the DP501 encoder's firmware. This
patch adds managed release of the allocated memory.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-9-tzimmermann@suse.de