drm/i915: Check for a second VCS engine more carefully

To use the legacy BSD selector, you must have a second VCS engine, or
else the ABI simply maps the request for another engine onto VCS0.
However, we only checked a single VCS1 location and overlooking the
possibility of a sparse VCS set being mapped to the dense ABI.

v2: num_vcs_engines() turns out to be reusable and futureproof it so we
never have to worry about this silly bit of ABI again!
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809123153.20574-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -2102,6 +2102,12 @@ static int eb_submit(struct i915_execbuffer *eb)
 	return 0;
 }
 
+static int num_vcs_engines(const struct drm_i915_private *i915)
+{
+	return hweight64(INTEL_INFO(i915)->engine_mask &
+			 GENMASK_ULL(VCS0 + I915_MAX_VCS - 1, VCS0));
+}
+
 /*
  * Find one BSD ring to dispatch the corresponding BSD command.
  * The engine index is returned.
@@ -2200,7 +2206,7 @@ eb_select_legacy_ring(struct i915_execbuffer *eb,
 		return -1;
 	}
 
-	if (user_ring_id == I915_EXEC_BSD && HAS_ENGINE(i915, VCS1)) {
+	if (user_ring_id == I915_EXEC_BSD && num_vcs_engines(i915) > 1) {
 		unsigned int bsd_idx = args->flags & I915_EXEC_BSD_MASK;
 
 		if (bsd_idx == I915_EXEC_BSD_DEFAULT) {