drm/i915: Emit even number of dwords when emitting LRIs

drm/i915: Emit even number of dwords when emitting LRIs

The number of DWords should be even when doing ring emits as
command sequences require QWord alignment.

There was some discussion about the maximum length of the MI_LRI
command. Quoting Mika

"I did some test with bdw:

"The maximum is 128 writes, resulting the 8 bit length
field of the command being 0xff, thus following the spec.
The 128'th write went through.

"Perhaps the max command length is then less in older gens?

"Perhaps WARN_ON(x > 128) in MI_LOAD_REGISTER_IMM would be in place
but one needs minor tweak to command parser a bit also then.

	#define I915_MAX_WA_REGS 16

keeps us safe for now atleast."

Ville commented that on pre-gen6 the length field seems to be
restricted to 0x3f though. So for all cases we should be ok.

v2: user LRI variant that can write multiple regs in one go (Damien).
We can simply insert one NOP at the end instead of one per register write.

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Add a summary of the MI_LRI length discussion.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>