drm/i915: Prefer IS_GEN<n> check with bitmask.

Whenever possible we should stick with IS_GEN<n> checks.

Bitmaks has been introduced on commit ae7617f0ef ("drm/i915:
Allow optimized platform checks") for efficiency.

Let's stick with it whenever possible.

This patch was generated with coccinelle:

spatch -sp_file is_gen.cocci *{c,h} --in-place

is_gen.cocci:
@gen2@ expression e; @@
-INTEL_GEN(e) == 2
+IS_GEN2(e)
@gen3@ expression e; @@
-INTEL_GEN(e) == 3
+IS_GEN3(e)
@gen4@ expression e; @@
-INTEL_GEN(e) == 4
+IS_GEN4(e)
@gen5@ expression e; @@
-INTEL_GEN(e) == 5
+IS_GEN5(e)
@gen6@ expression e; @@
-INTEL_GEN(e) == 6
+IS_GEN6(e)
@gen7@ expression e; @@
-INTEL_GEN(e) == 7
+IS_GEN7(e)
@gen8@ expression e; @@
-INTEL_GEN(e) == 8
+IS_GEN8(e)
@gen9@ expression e; @@
-INTEL_GEN(e) == 9
+IS_GEN9(e)
@gen10@ expression e; @@
-INTEL_GEN(e) == 10
+IS_GEN10(e)
@gen11@ expression e; @@
-INTEL_GEN(e) == 11
+IS_GEN11(e)

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026195143.20353-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4741,13 +4741,13 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 			selected_result = method2;
 		} else if (ddb_allocation >=
 			 fixed16_to_u32_round_up(wp->plane_blocks_per_line)) {
-			if (INTEL_GEN(dev_priv) == 9 &&
+			if (IS_GEN9(dev_priv) &&
 			    !IS_GEMINILAKE(dev_priv))
 				selected_result = min_fixed16(method1, method2);
 			else
 				selected_result = method2;
 		} else if (latency >= wp->linetime_us) {
-			if (INTEL_GEN(dev_priv) == 9 &&
+			if (IS_GEN9(dev_priv) &&
 			    !IS_GEMINILAKE(dev_priv))
 				selected_result = min_fixed16(method1, method2);
 			else