i965/fs: Fetch one cacheline of pull constants at a time.

i965/fs: Fetch one cacheline of pull constants at a time.

Asking the DC for less than one cacheline (4 owords) of data for
uniform pull constants is suboptimal because the DC cannot request
less than that from L3, resulting in wasted bandwidth and unnecessary
message dispatch overhead, and exacerbating the IVB L3 serialization
bug.  The following table summarizes the overall framerate improvement
(with statistical significance of 5% and sample size ~10) from the
whole series up to this patch for several benchmarks and hardware
generations:

                         | SKL           | BDW          | HSW
SynMark2 OglShMapPcf     | 24.63% ±0.45% | 4.01% ±0.70% | 10.31% ±0.38%
GfxBench4 gl_manhattan31 |  5.93% ±0.35% | 3.92% ±0.31% |  6.62% ±0.22%
GfxBench4 gl_4           |  2.52% ±0.44% | 1.23% ±0.10% |      N/A
Unigine Valley           |  0.83% ±0.17% | 0.23% ±0.05% |  0.74% ±0.45%

Note that there are two versions of the Manhattan demo shipped with
GfxBench4, one of them is the original gl_manhattan demo which doesn't
use UBOs, so this patch will have no effect on it, and another one is
the gl_manhattan31 demo based on GL 4.3/GLES 3.1, which this patch
benefits as shown above.

I haven't observed any statistically significant regressions in the
benchmarks I have at hand.  Note that the comparatively huge
improvement on SKL in the OglShMapPcf test case is due to the combined
effect of this patch and the register pressure benefit on SKL+ of
"i965/fs: Switch to the constant cache for uniform pull constants.",
part of the same series.

Going up to 8 oword blocks would improve performance of pull constants
even more, but at the cost of some additional bandwidth and register
pressure, so it would have to be done on-demand based on the number of
constants actually used by the shader.

v2: Fix for Gen4 and 5.
v3: Non-trivial rebase.  Rework to allow the visitor specifiy
    arbitrary pull constant block sizes.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>