drm/i915: Simplify vcs/bsd engine selection

No need to look at the mask of present engines when we already have a
count stored ever since e2d0ff35

 ("drm/i915: Count engine instances
per uabi class").

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316142728.1335239-1-tvrtko.ursulin@linux.intel.com
[tursulin: fixup typo in patch title]