drm/i915: Update wa_ctx_emit() macro as per kernel coding guidelines

wa_ctx_emit() depends on the name of a local variable; if the name of that
variable is changed then we get compile errors. In this case it is unlikely
to be changed as this macro is only used in this set of functions but
Kernel coding guidelines doesn't recommend doing this. It was my mistake
as I should have corrected it at the beginning but missed so correct
this before there are more usages of this macro (Bob Beckett).

https://www.kernel.org/doc/Documentation/CodingStyle

,
Chapter 12, "Things to avoid when using macros", point 2):

"
2) macros that depend on having a local variable with a magic name:

   #define FOO(val) bar(index, val)

might look like a good thing, but it's confusing as hell when one reads the
code and it's prone to breakage from seemingly innocent changes.
"

v2: Optimization to avoid multiple evaluation of 'index' in the macro.
Since we invoke it multiple times, compiler, if it can, should be able to coalesce
them into a single condition and remove multiple WARN_ON checks (Chris).

Suggested-by: Robert Beckett <robert.beckett@intel.com>
Cc: Robert Beckett <robert.beckett@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>