drm/i915: Fix busy ioctl commentary

Just tidy one instance of incorrect context parameter name and a stray
sentence ending from before reporting was converted to be class based.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611132221.1055650-1-tvrtko.ursulin@linux.intel.com