drm: mali-dp: Check the mclk rate and allow up/down scaling

When downscaling, mclk needs to be sufficiently higher than pxlclk in
order to be able to fetch the higher-resolution data and produce output
pixels. When not scaling, or when upscaling, mclk can be equal to
pxlclk. Since the driver doesn't control mclk, just ensure that the
requirement is satisfied with the current clock rate.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>