drm/i915: Clean up the ring scaling calculations

This patch attempts to clean up the ring/IA scaling programming in the
following ways.
1. Fix the comment about the DDR frequency. The math is 266MHz, not
133MHz. Formula was right, docs are wrong.

2. Mask the DCLK register since I don't know how it is defined on future
platforms.

3. use mult_frac instead of magic math.

This helps for future platform enabling.

v2: Actually use the right patch. The v1 was a mix of things, none of
which was right. Note that due to rounding, we actually get different
values (slightly higher) for the effective ring frequency.

v3: Use 1.25 instead of 1.33 as the original code did. (Jesse)

CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>