drm/i915: Give g4x_{dp,hdmi}.c g4x_ namespace

s/intel_/g4x_/ for the externally visible g4x_{dp,hdmi}.c
functions.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-8-ville.syrjala@linux.intel.com