drm/i915: fix a couple of spelling mistakes in kernel parameter help text

There are a couple of spelling mistakes in kernel parameter help text,
namely "helpfull" and "paramters".  Fix them.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616082129.65517-1-colin.king@canonical.com