i965: Enable ARB_transform_feedback2 on Gen7+ if register writes work.

i965: Enable ARB_transform_feedback2 on Gen7+ if register writes work.

With Linux 3.12, register writes work on Ivybridge and Baytrail, but not
Haswell.  That will be fixed in a future kernel revision, at which point
this extension will automatically be enabled.

v2: Use I915_GEM_DOMAIN_INSTRUCTION for the register read, and also
    correctly set the writeable flag when mapping (caught by Eric).

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