drm/i915/cnl: Add Gen10 LRC size

The total size of the context has decreased with the removal of the
URB_ATOMIC section. BSpec indicates 16750 DWORDs (17 pages), plus
one page for PPHWSP, and I'm throwing an extra page for precaution.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1506035989-14295-1-git-send-email-oscar.mateo@intel.com