drm/i915: Delete outdated comment in byt_pte_encode

This has been invalidated in

commit 24f3a8cf
Author: Akash Goel <akash.goel@intel.com>
Date:   Tue Jun 17 10:59:42 2014 +0530

    drm/i915: Added write-enable pte bit supportt

But despite that it's in the diff context no one noticed :(

Cc: Akash Goel <akash.goel@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -164,9 +164,6 @@ static gen6_gtt_pte_t byt_pte_encode(dma_addr_t addr,
 	gen6_gtt_pte_t pte = valid ? GEN6_PTE_VALID : 0;
 	pte |= GEN6_PTE_ADDR_ENCODE(addr);
 
-	/* Mark the page as writeable.  Other platforms don't have a
-	 * setting for read-only/writable, so this matches that behavior.
-	 */
 	if (!(flags & PTE_READ_ONLY))
 		pte |= BYT_PTE_WRITEABLE;