drm/i915/lt_phy: Add check if PLL is enabled

drm/i915/lt_phy: Add check if PLL is enabled

Add check for PLL enabling and return early if
PLL is not enabled.

v2: Use PCLK PLL ACK bit to check if PLL is enabled (Suraj)
v3: Check only if PCLK PLL ACK bit for lane 0 is enabled (Suraj)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/20260312080657.2648265-3-mika.kahola@intel.com