drm/etnaviv: switch to PFN mappings

There is no reason to use page based mappings, as the established
mappings are special driver mappings anyways and should not be
handled like normal pages.

Be consistent with what other drivers do and use raw PFN based
mappings.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>