drm: etnaviv: clean up vram_mapping submission/retire path

Currently, we scan the list of mappings each time we want to operate on
the vram_mapping struct.  Rather than repeatedly scanning these, look
them up once in the submission path, and then use _reference and
_unreference methods as necessary to manage this object.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>