drm/i915: Rename IS_GEN to IS_GEN_RANGE

RANGE makes it longer, but clearer. We are also going to add a macro to
check an individual gen, so add the _RANGE prefix here.

Diff generated with:

sed 's/IS_GEN(/IS_GEN_RANGE(/g' drivers/gpu/drm/i915/{*/,}*.{c,h} -i

v2: use IS_GEN rather than GT_GEN

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181212181044.15886-1-lucas.demarchi@intel.com