drm/i915: Move context descriptor fields to intel_lrc.h

This is a more appropriate header for these definitions.

v2:
 - Cleanup whitespace. (Lucas)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220301231549.1817978-7-matthew.d.roper@intel.com