drm/i915: fix freeze with blank screen booting highmem

drm/i915: fix freeze with blank screen booting highmem

x86_64 boots and displays fine, but booting x86_32 with CONFIG_HIGHMEM
has frozen with a blank screen throughout 3.16-rc on this ThinkPad T420s,
with i915 generation 6 graphics.

Fix 75107cf4eb22 ("drm/i915: add render state initialization"): kunmap()
takes struct page * argument, not virtual address.  Which the compiler
kindly points out, if you use the appropriate u32 *batch, instead of
silencing it with a void *.

Why did bisection lead decisively to nearby c58deabe48dd ("drm/i915:
add null render states for gen6, gen7 and gen8")?  Because the u32
deposited at that virtual address by the previous stub failed the
PageHighMem test, and so did no harm.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>