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>

include: Move ascii85 functions from i915 to linux/ascii85.h

The i915 DRM driver very cleverly used ascii85 encoding for their
GPU state file. Move the encode functions to a general header file to
support other drivers that might be interested in the same
functionality.

v4: Make the return value const char * as suggested by Chris Wilson
v3: Fix error_puts -> err_puts pointed out by the 01.org bot
v2: Update API to be cleaner for the caller as suggested by Chris Wilson

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>