drm/i915/dp: Make dsc helpers accept const crtc_state pointers

Modify the dsc helpers to get max/min compressed bpp to accept
`const struct intel_crtc_state *` pointers instead of
`struct intel_crtc_state *`.

These helpers are not supposed to modify `crtc_state`.
Accepting const pointers will allow these helpers to be called from
functions that have const pointer to crtc_state.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241217093244.3938132-13-ankit.k.nautiyal@intel.com