drm/i915: Kill duplicated PNV .get_display_clock_speed() assignment

drm/i915: Kill duplicated PNV .get_display_clock_speed() assignment

Somehow we accumulated a duplicated .get_display_clock_speed()
assignment for PNV in
commit 34edce2fea69 ("drm/i915: Add cdclk extraction for g33, g965gm and g4x")

No real harm on having two, we just never reach the second one, so
simply kill it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448893432-6978-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>