i915: cast before shifting in i915_pte_count

Otherwise a pde_shift big enough to overflow a u32 will be truncated before
assignment

Note: We never asked for ranges spanning a 4G boundary, so this issue
doesn't cause a real problem.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
[danvet: Add note why this isn't a real problem.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160217142043.4947.60447.stgit@localhost.localdomain

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -418,7 +418,7 @@ static inline uint32_t i915_pte_index(uint64_t address, uint32_t pde_shift)
 static inline uint32_t i915_pte_count(uint64_t addr, size_t length,
 				      uint32_t pde_shift)
 {
-	const uint64_t mask = ~((1 << pde_shift) - 1);
+	const uint64_t mask = ~((1ULL << pde_shift) - 1);
 	uint64_t end;
 
 	WARN_ON(length == 0);