drm/i915/dsi: use vlv and bxt prefixes for the global DSI functions

Avoid confusion with the functions to be added for the new ICL or gen 11
DSI implementation by renaming the current DSI functions. While at it,
permutate the words in the function names to make them all start with
"vlv_dsi" or "vlv_dsi_pll" etc.

Reduce the platform abstractions in the PLL file while at it, moving the
checks to vlv_dsi.c instead, where we typically already have the
necessary if ladders.

Leave the static functions as-is for now; they could be renamed later if
needed.

No functional changes.

v2: use "gen7" prefix.

v3: use "vlv" and "bxt" prefixes, reduce the abstractions.

References: https://patchwork.freedesktop.org/series/44823/
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705132509.12881-2-jani.nikula@intel.com