drm/i915/cnl: Avoid ioremap_wc on Cannonlake as well.

Driver’s CPU access to GTT is via the GTTMMADR BAR.

The current HW implementation of that BAR is to only
support <= DW (and maybe QW) writes—not 16/32/64B writes
that could occur with WC and/or SSE/AVX moves.

GTTMMADR must be marked uncacheable (UC).
Accesses to GTTMMADR(GTT), must be 64 bits or less (ie. 1 GTT entry).

v2: Get clarification on the reasons and spec is getting
    updated to reflect it now.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Suggested-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829230907.21363-1-rodrigo.vivi@intel.com