drm/i915/gt: Replace manual kmap_atomic() with pin_map for renderstate

We only emit the renderstate once now during module load, it is no
longer a concern that we are delaying context creation and so do not
need to so eagerly optimise. Since the last time we have looked at the
renderstate, we have a pin_map / flush_map facility that supports simple
single mappings, replacing the open-coded kmap_atomic() and
prepare_write. As it should be a single page, of which we only write a
small portion, we stick to a simple WB [kmap] and use clflush on !llc
platforms, rather than creating a temporary WC vmapping for the single
page.

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/20200619234543.17499-2-chris@chris-wilson.co.uk