drm/i915/dp: Factor out intel_dp_read_dprx_caps()

Factor out a function to read the sink's DPRX capabilities used by a
follow-up patch enabling the DP tunnel BW allocation mode.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240220211841.448846-10-imre.deak@intel.com