drm/i915/dp: Add helper to compute num pipes required

Add a helper to compute the number of pipes required.
This will depend on whether the joiner is required or is forced through
the debugfs. If no joiner is required the helper returns 1.

v2:
-Return 1 if no joiner is required. (Ville)
-Change the suffix from joined_pipes to num_pipes. (Ville)
-Use number of pipes while calculating joined_pipe masks and
max_dotclk. (Ville)
v3: Simplify and rename the helper to intel_dp_num_joined_pipes(). Ville
v4: Remove redundant 'fallthrough' statement. (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/20240926134322.3728021-5-ankit.k.nautiyal@intel.com