drm/i915/alpm: Make crtc_state as const in intel_alpm_compute_params

Intel_alpm_compute_params doesn't change crtc_state. Let's convert it as
const.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619062131.4021196-8-jouni.hogander@intel.com