iris: Enable PIPE_CAP_CS_DERIVED_SYSTEM_VALUES_SUPPORTED

iris: Enable PIPE_CAP_CS_DERIVED_SYSTEM_VALUES_SUPPORTED

This avoids lowering of CS system values by GLSL (configured by state
tracker).  In i965 we don't use that lowering, and we also shouldn't
need that in Iris.

Using it cause some unnecessary round trip between values, e.g.:
shader uses gl_LocalInvocationIndex, GLSL rewrites it in terms of
gl_LocalInvocationID, then driver rewrites those in terms of
gl_LocalInvocationIndex again.  Copy propagation can make some of
those go away, but not all as seen below.

Intel SKL shader-db results:

    total instructions in shared programs: 15595189 -> 15594556 (<.01%)
    instructions in affected programs: 74880 -> 74247 (-0.85%)
    helped: 81
    HURT: 4
    helped stats (abs) min: 2 max: 172 x̄: 7.88 x̃: 4
    helped stats (rel) min: 0.19% max: 5.66% x̄: 1.71% x̃: 1.23%
    HURT stats (abs)   min: 1 max: 2 x̄: 1.25 x̃: 1
    HURT stats (rel)   min: 0.45% max: 1.65% x̄: 0.76% x̃: 0.46%
    95% mean confidence interval for instructions value: -11.56 -3.34
    95% mean confidence interval for instructions %-change: -1.91% -1.28%
    Instructions are helped.

    total loops in shared programs: 4831 -> 4831 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total cycles in shared programs: 372136618 -> 372145628 (<.01%)
    cycles in affected programs: 9218230 -> 9227240 (0.10%)
    helped: 131
    HURT: 86
    helped stats (abs) min: 1 max: 798 x̄: 39.79 x̃: 12
    helped stats (rel) min: <.01% max: 6.75% x̄: 0.42% x̃: 0.13%
    HURT stats (abs)   min: 2 max: 2442 x̄: 165.38 x̃: 6
    HURT stats (rel)   min: <.01% max: 20.83% x̄: 0.74% x̃: 0.12%
    95% mean confidence interval for cycles value: -2.07 85.11
    95% mean confidence interval for cycles %-change: -0.22% 0.30%
    Inconclusive result (value mean confidence interval includes 0).

    total spills in shared programs: 11956 -> 11950 (-0.05%)
    spills in affected programs: 77 -> 71 (-7.79%)
    helped: 3
    HURT: 0

    total fills in shared programs: 25619 -> 25549 (-0.27%)
    fills in affected programs: 593 -> 523 (-11.80%)
    helped: 4
    HURT: 0

    LOST:   0
    GAINED: 0

    Total CPU time (seconds): 1695.69 -> 1706.03 (0.61%)

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>