drm/i915: Turn off pipe gamma when it's not needed

The pipe internal precision is higher than what we currently program to
the degamma/gamma LUTs. We can get a higher quality image by bypassing
the LUTs when they're not needed. Let's do that.

Each plane has its own control bit for this, so we have to update
all active planes. The way we've done this we don't actually have
to run through the whole .check_plane() thing. And we actually
do the .color_check() after .check_plane() so we couldn't even do
that without shuffling the code around.

Additionally on pre-skl we have to update the primary plane regardless
of whether it's active or not on account of the primary plane gamma
enable bit also affecting the pipe bottom color.

v2: Drop the '.' from patch title (Uma)
    Fix 'primayr' typo (Uma,Matt)
    Rebase

Signed-off-by: Ville Syrjälä <[email protected]>
Reviewed-by: Uma Shankar <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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