video: ipu_disp: squash clang warning

video: ipu_disp: squash clang warning

Since rgb2ycbcr_coeff and friends are declared const, but assigned
to a void pointer, clang will warn that the const is implicity casted
away. If the pointer is changed to void const * gcc will warn when it
is implicitly casted to a const int array. Just add a correctly
typed pointer instead to prevent these casts and hence the warnings.

Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>