drm/i915: Add support for forcing the link bpp on a connector

drm/i915: Add support for forcing the link bpp on a connector

Add support for forcing the link bpp on a connector via a connector
debugfs entry. During reducing link bpps due to a link BW limit, keep
bpps close to their forced value.

Add the debugfs entry to all relevant connectors: all DP connectors and
on an FDI link CRT/SDVO/LVDS/HDMI connectors.

v2:
- Move adding the debugfs entries to this patch.
- Rename i915_force_link_bpp to intel_force_link_bpp. (Jani)
- Select the relevant connectors via platform checks. (Jani)
- Use for_each_new_intel_connector_in_state(). (Jani)
- Fix 64 bit division vs. 32 bit build when converting str to q4. (lkp)
- Avoid division and addition overflow when converting str to q4.

v3:
- Add TODO: to make the non-DSC min bpp value connector specific. (Ankit)

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-12-imre.deak@intel.com