drm/i915/glk, cnl: Implement WaDisableScalarClockGating

On GLK and CNL enabling a pipe with its pipe scaler enabled will result
in a FIFO underrun. This happens only once after driver loading or
system/runtime resume, more specifically after power well 1 gets
enabled; subsequent modesets seem to be free of underruns. The BSpec
workaround for this is to disable the pipe scaler clock gating for the
duration of modeset. Based on my tests disabling clock gating must be
done before enabling pipe scaling and we can re-enable it after the pipe
is enabled and one vblank has passed.

For consistency I also checked if plane scaling would cause the same
problem, but that doesn't seem to trigger this problem.

The patch is based on an earlier version from Ander.

v2 (Rodrigo):
- Set also CLKGATE_DIS_PSL bits 8 and 9.
- Add also the BSpec workaround ID.

Cc: Ander Conselvan de Oliveira <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100302
Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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