i965: Make some EU emit code for DP read/write messages non-static.

i965: Make some EU emit code for DP read/write messages non-static.

We keep building these strange interfaces for DP read/write where
there's a helper function with some partially-specific,
partially-general controls, which is used in exactly one place in code
generation.  Making these public will let us set up those instructions
in the one place they're to be generated.