drm/i915: FIFO watermark calculation fixes

I discovered several bugs in the FIFO code that was recently applied.
Some of them fell into the "how did this ever work" category, since in
some cases we were using the wrong FIFO size values, and the
calculations ended up being way off.

This patch fixes all the bugs I found, and works well on my GM45, 915GM
and 855GM test machines; but as usual with these sorts of patches
broader testing is definitely requested (in particular this patch
affects 830, 845 and 865 for which I don't have test hardware).

Overall, the patch clarifies the watermark calculation function by
adding some comments and debug info, and making the variable names a
bit clearer.  The "get FIFO size" portion of the code has also been
corrected, so we should be able to properly detect the FIFO allocations
for each pipe, for use in the watermark calculation.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>