drm: mali-dp: Rename malidp_input_format to malidp_pixel_format

We're going to use the same format list for output formats, so rename
everything related to input formats to avoid confusion.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
[touched commit title to clarify the final struct name]
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>