i965: Pull most driconf option handling into a centralized function.

i965: Pull most driconf option handling into a centralized function.

Using a helper function clarifies the context initialization code.

I would've liked to completely centralize it, but moving the optionCache
code from intelInitExtensions into here would've required setting flags
in the context, which seems like a waste.

v2: Rebase for the introduction of disable_derivative_optimization.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>