Every set_pages_array_wb call resulted in cross-core
interrupts and TLB flushes. Merge more of them for
less overhead.

This reduces the time needed to free a 1.6 GiB GTT WC
buffer as part of Vulkan CTS from  ~2 sec to < 0.25 sec.
(Allocation still takes more than 2 sec though)

(v2): use set_pages_wb instead of set_memory_wb.
Signed-off-by: Bas Nieuwenhuizen <basni@chromium.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>