drm/i915: stop setting cache_dirty on discrete

Should not be needed. Even with non-coherent display, we should be using
device local-memory there, and not system memory.

v2: also add a warning in i915_gem_clflush_object

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20211027161813.3094681-4-matthew.auld@intel.com