drm/i915/bdw: New source and header file for LRs, LRCs and Execlists

Some legacy HW context code assumptions don't make sense for this new
submission method, so we will place this stuff in a separate file.

Note for reviewers: I've carefully considered the best name for this file
and this was my best option (other possibilities were intel_lr_context.c
or intel_execlist.c). I am open to a certain bikeshedding on this matter,
anyway.

And some point in time, it would be a good idea to split intel_lrc.c/.h
even further, but for the moment just shove everything together.

v2: Change to intel_lrc.c

v3: Squash together with the header file addition

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>