drm/i915: Move LRC register offsets to a header file

Newer platforms may have subtle offset changes, which will increase the
number of defines, so it is probably better to start moving them to its
own header file. Also move the macros used while setting the reg state.

v2: Rename to intel_lrc_reg.h, to be consistent with i915_reg.h and
intel_guc_reg.h (Chris)
v3: License notice shenanigans.
v4: Documentation/process/coding-style.rst is always right (Chris)
v5: Rebase.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180124004349.22126-2-michel.thierry@intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>