drm/i915/cnl: Do not add an extra page for precaution in the Gen10 LRC size

BSpec indicates exactly 16752 DWORDs (17 pages), plus one page for PPHWSP. Please
notice that, when looking at the BSpec context image table, the right filter has
to be applied (e.g. "CNL") as some rows are excluded for specific GENs.

BSpec: 1383

v2: Update count and add BSpec tag (Joonas)
v3: Warning about filters in the commit message (Joonas)
Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Fixes: 7fd0b1a2 ("drm/i915/cnl: Add Gen10 LRC size")
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>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507131592-29209-1-git-send-email-oscar.mateo@intel.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -39,7 +39,7 @@
 
 #define GEN8_LR_CONTEXT_RENDER_SIZE	(20 * PAGE_SIZE)
 #define GEN9_LR_CONTEXT_RENDER_SIZE	(22 * PAGE_SIZE)
-#define GEN10_LR_CONTEXT_RENDER_SIZE	(19 * PAGE_SIZE)
+#define GEN10_LR_CONTEXT_RENDER_SIZE	(18 * PAGE_SIZE)
 
 #define GEN8_LR_CONTEXT_OTHER_SIZE	( 2 * PAGE_SIZE)