drm/i915/vblank: Make the vblank counter u64 -> u32 typecast explicit

Core returns a u64 vblank count and intel_crtc_get_vblank_counter()
expects a 32-bit value. Make the typecast explicit to add clarity.

Cc: Keith Packard <keithp@keithp.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-2-dhinakaran.pandiyan@intel.com