drm/i915/display/dsc: Add Per connector debugfs node for DSC BPP enable

This patch creates a per connector debugfs node to expose
the Compressed BPP. The same node can be used from userspace
to force DSC to a certain BPP(all accepted values).

This is useful to verify all supported/requested
compression bpp's through IGT

v2: Remove unnecessary logic (Jani)
v3: Drop pipe bpp in debugfs node (Vandita)
v4: Minor cleanups (Vandita)
v5: Fix NULL pointer dereference
v6: Fix dim tool checkpatch errors
    Release the lock before return (Vandita)
v7: Rename to file to dsc_bpp, remove unwanted
    dsc bpp range check from v6, permissions (Jani)

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Navare Manasi D <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Patnana Venkata Sai <venkata.sai.patnana@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720064907.9771-3-vandita.kulkarni@intel.com