drm/i915: Rename PIPE_CONTROL bit defines to be less terse.

"STALL_AT_SCOREBOARD" is much clearer than "STALL_EN" now that there are
several different kinds of stalls.  Also, "INSTRUCTION_CACHE_INVALIDATE"
is a lot easier to understand at a glance than the terse "IS_FLUSH."

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: use INVALIDATE for ro cache flags for more consistency]
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>