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: Alan Cox <alan@linux.intel.com>
[danvet: Add note why this isn't a real problem.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160217142043.4947.60447.stgit@localhost.localdomain