drm/i915: add helpers for platform specific revision id range checks

Revision checks are almost always accompanied by a platform check. (The
exceptions are platform specific code.) Add helpers to check for a
platform and a revision range: IS_SKL_REVID() and IS_BXT_REVID(). In
most places this simplifies and clarifies the code. It will be obvious
that revid macros are used for the correct platform.

This should make it easier to find all the revision checks for
workarounds for each platform, and make it easier to remove them once we
drop support for early hardware revisions.

This should also make it easier to differentiate between Skylake and
Kabylake revision checks when Kabylake support is added.

v2: rebase

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445343722-3312-3-git-send-email-jani.nikula@intel.com