drm/i915/display: Use HAS_LT_PHY() for LT PHY AUX power

drm/i915/display: Use HAS_LT_PHY() for LT PHY AUX power

Bspec states that the new AUX power enable/disable sequences are
associated with the LT PHY.  As such, use HAS_LT_PHY() instead of IP
checks in those paths in the driver code.

While at it, also move the comment that we can't use the power status
flag to the "else" branch, since that comment is not applicable for the
LT PHY.

Bspec: 68967
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/20251202012306.9315-9-matthew.s.atwood@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>