i965: Don't reallocate push constant URB space on new VS programs.

The gen7_urb atom depends on CACHE_NEW_VS_PROG and CACHE_NEW_GS_PROG,
causing gen7_upload_urb() to be called when switching to a new VS
program.

In addition to partitioning the URB space between the VS and GS,
gen7_upload_urb() also allocated space for VS and PS push constants.
Unfortunately, this meant that whenever CACHE_NEW_VS was flagged, we'd
reallocate the space for the PS push constants.  According to the BSpec,
after sending 3DSTATE_PUSH_CONSTANT_ALLOC_PS, we must reprogram
3DSTATE_CONSTANT_PS prior to the next 3DPRIMITIVE.

Since our URB allocation for push constants is entirely static, it makes
sense to split it out into its own atom that only subscribes to
BRW_NEW_CONTEXT.  This avoids reallocating the space and trashing
constants.

Fixes a rendering artifact in Extreme Tuxracer, where instead of a snow
trail, you'd get a bright red streak (affectionately known as the
"bloody penguin bug").

This also explains why adding VS-related dirty bits to gen7_ps_state
made the problem disappear: it made 3DSTATE_CONSTANT_PS be emitted after
every 3DSTATE_PUSH_CONSTANT_ALLOC_PS packet.

NOTE: This is a candidate for the 7.11 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38868
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

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