drm/i915/dsi: merge pre_pll_enable hook to pre_enable

For DSI, the pre_pll_enable and the pre_enable hooks are called
back-to-back on all platforms that support DSI. The distinction is
artificial for DSI, for which we enable the DSI PLL in the encoder
hooks. Do everything in pre_enable, and remove DSI pre_pll_enable hook.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448619706-21293-1-git-send-email-jani.nikula@intel.com