drm/i915: Move dsc rate params compute into drm

The function intel_compute_rc_parameters is part of the dsc spec
and is not driver-specific. Other drm drivers might like to use
it.  The function is not changed; just moved and renamed.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190221202001.28430-2-David.Francis@amd.com