i965: Disable clipper statistics when meta operations are in progress.

i965: Disable clipper statistics when meta operations are in progress.

We don't currently use the clipper statistics, but we'll soon use
CL_INVOCATIONS_COUNT to implement the GL_PRIMITIVES_GENERATED query.
The number of primitives generated is not supposed to be altered during
operations such as glGenerateMipmap.

Prevents spec/EXT_transform_feedback/generatemipmap prims_generated
from breaking when we start using pipeline statistics registers to
implement the GL_PRIMITIVES_GENERATED query in a few commits.

v2: Use the BRW_NEW_META_IN_PROGRESS flag for correct state handling.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> [v1]
Reviewed-by: Paul Berry <stereotype441@gmail.com>