drm/amd/powerplay: fix spelling mistake and add KERN_WARNING to printks

Fix trivial spelling mistake cant't -> can't and add KERN_WARNING to
printk messages.  Remove redundant spaces before \n too (thanks to
Joe Perches for spotting those).

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>