drm/i915: clarify that checking the FB stride for CFB is intentional

Daniel was looking at this code and asked about whether fb->pitches[0]
is correct, then he suggested we should a comment to make sure it is
actually intentional.

For more information on the CFB size calculation, please see the
commit message of:

commit c4ffd40
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Thu Oct 1 19:55:57 2015 -0300
    drm/i915: fix CFB size calculation

Requested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446664257-32012-12-git-send-email-paulo.r.zanoni@intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_fbc.c

@@ -707,6 +707,7 @@ static int intel_fbc_calculate_cfb_size(struct intel_crtc *crtc)
 	if (INTEL_INFO(dev_priv)->gen >= 7)
 		lines = min(lines, 2048);
 
+	/* Hardware needs the full buffer stride, not just the active area. */
 	return lines * fb->pitches[0];
 }