It is possible for a mode set to fail if there aren't shared DPLLS that
match the new configuration requirement or other errors in clock
computation. If that step is executed after disabling crtcs, in the
failure case the hardware configuration is changed and needs to be
restored. Doing those things early will allow the mode set to fail
before actually touching the hardware.

Follow up patches will convert different platforms to use the new
infrastructure.

v2: Keep pll->new_config valid only during mode set (Ville)
    Use kmemdup() in i915_shared_dpll_start_config() (Ville)
    Restore old pll config if something fails before commit (Ville)
    Don't set compute_clock hooks since dev_priv is kzalloc()'d (Ville)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>