drm: include only needed headers in ascii85.h

The ascii85.h is user of exactly two headers, i.e.  math.h and types.h.
There is no need to carry on entire kernel.h.

Link: https://lkml.kernel.org/r/20210611185915.44181-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>