drm/i915/display: Modify debugfs for joiner to force n pipes

At the moment, the debugfs for joiner allows only to force enable/disable
pipe joiner for 2 pipes. Modify it to force join 'n' number of pipes,
where n is a valid pipe joiner configuration.
This will help in case of ultra joiner where 4 pipes are joined.

v2:
-Fix commit message to state that only valid joiner config can be
forced. (Suraj)
-Rename the identifiers to have INTEL_BIG/NONE_JOINER_PIPES. (Suraj)
v3:
-Avoid enum for joiner pipe counts, use bare numbers for better
readability. (Ville)
-Remove redundant prints from debugfs. (Ville)
v4: Return -EINVAL if joiner forced to an invalid value.
v5: Remove extra debug message. (Ville)
v6: Minor fix in switch case. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240926134322.3728021-4-ankit.k.nautiyal@intel.com