drm/i915: SAGV is not SKL-only, so rename a few things

drm/i915: SAGV is not SKL-only, so rename a few things

The plan is to introduce intel_has_sagv() and then use it to discover
which platforms actually support it.

I thought about keeping the functions with their current skl names,
but found two problems: (i) skl_has_sagv() would become a very
confusing name, and (ii) intel_atomic_commit_tail() doesn't seem to be
calling any functions whose name start with a platform name, so the
"intel_" naming scheme seems make more sense than the "firstplatorm_"
naming scheme here.

Cc: stable@vger.kernel.org
Reviewed-by: Lyude <cpaul@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-2-git-send-email-paulo.r.zanoni@intel.com
(cherry picked from commit 16dcdc4edbcf5cb130004737f2548401776170f1)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>