drm/i915: Fix enabled_planes bitmask

drm/i915: Fix enabled_planes bitmask

The enabled_planes bitmask was supposed to track logically enabled
planes (ie. fb!=NULL and crtc!=NULL), but instead we end up putting
even disabled planes into the bitmask since
intel_plane_atomic_check_with_state() only takes the early exit
if the plane was disabled and stays disabled. I think I misread
the early said codepath to exit whenever the plane is logically
disabled, which is not true.

So let's fix this up properly and set the bit only when the plane
actually is logically enabled.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Fixes: ee42ec19ca2e ("drm/i915: Track logically enabled planes for hw state")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-2-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>