drm/bridge: dw-hdmi: rework csc related functions

is_color_space_conversion() is a misnomer. It checks not only if color
space conversion is needed, but also if format conversion is needed.
This is actually desired behaviour because result of this function
determines if CSC block should be enabled or not (CSC block can also do
format conversion).

In order to clear misunderstandings, let's rework
is_color_space_conversion() to do exactly what is supposed to do and add
another function which will determine if CSC block must be enabled or
not.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304232512.51616-5-jernej.skrabec@siol.net