drm/i915: use consistent CPU mappings for pin_map users

For discrete, users of pin_map() needs to obey the same rules at the TTM
backend, where we map system only objects as WB, and everything else as
WC. The simplest for now is to just force the correct mapping type as
per the new rules for discrete.

Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210705135310.1502437-1-matthew.auld@intel.com