drm/i915: Store the pipe pixel rate in the crtc state

drm/i915: Store the pipe pixel rate in the crtc state

Rather than recomputing the pipe pixel rate on demand everywhere, let's
just stick the precomputed value into the crtc state.

v2: Rebase due to min_pixclk[] code movement
    Document the new pixel_rate struct member (Ander)
    Combine vlv/chv with bdw+ in intel_modeset_readout_hw_state()
v3: Fix typos in commit message (David)

Cc: Ander Conselvan De Oliveira <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Reviewed-by: Ander Conselvan de Oliveira <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]