drm/i915: Improve vlv_gpu_freq() and vlv_freq_opcode()

We're currently miscalculating the VLV graphics clock a little bit.
This is caused by rounding the step to integer MHz, which does not
match reality. Change the formula to match the GUnit HAS to give
more accurate answers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>