drm/i915: Tidy execlists_init_reg_state

Compact the name of the macro and reg_state variable, and cache
some data in local variables to make the function more compact
and more readable.

v2: Fixup some checkpatch warnings.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170221095839.30525-1-tvrtko.ursulin@linux.intel.com