drm/i915/gvt: Drop redundant prepare_write/pin_pages

Since gvt calls pin_map for the shadow batch buffer, this makes the
action of prepare_write [+pin_pages] redundant. We can write into the
obj->mm.mapping directory and the flush_map routine knows when it has to
flush the cpu cache afterwards.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200619234543.17499-1-chris@chris-wilson.co.uk