drm/i915: Assert dpll running in intel_crtc_load_lut() on pre-PCH platforms

Adding more context from Ville's reply to Rodrigo's question why we
need this:

"The spec says that on some hardware you need to PLL running before you
can poke at the palette registers. I didn't actually try to anger the
hardware so I'm not really sure what would happen otherwise, but IIRC
Jesse said something about a hard system hang..."

And generally documenting such ordering constraints with asserts is
Just Good.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Spruce up the commit message a lot.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>