drm/i915/utils: remove unused KBps/MBps/GBps macros

Remove unused macros. If needed again, such macros belong in
<linux/units.h>.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220815080907.3229449-1-jani.nikula@intel.com