intel: Move RenderMode fallback func to i915 driver.

intel: Move RenderMode fallback func to i915 driver.

The Fallback field of the context struct doesn't work that way on i965, and
it's the only caller of FALLBACK() in the driver.

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