drm: Fix 3 typos in the inline doc

Fix the following typos:

1. When mentioning a list of functions, the function
drm_atomic_helper_disable_plane is mentioned twice.

2. drop the word 'afterwards':
s/afterwards after that/after that/'

3. drop extra 'the':
s/but do not the support the full/but do not support the full/

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326103216.7918-1-dafna.hirschfeld@collabora.com