drm/i915/psr: clarify intel_psr_pre_plane_update() conditions

drm/i915/psr: clarify intel_psr_pre_plane_update() conditions

Make the conditions easier to follow. We don't do anything for
!psr->enabled, so hoist psr->enabled check higher, avoiding all the
checks when !psr->enabled. Stop the bitwise OR abuse on booleans by
removing the temporary variable altogether.

v2: Rebase

Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com> # v1
Link: https://patchwork.freedesktop.org/patch/msgid/20250204140518.2971530-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>