drm/amd/display: Fix typo in some comments

p and g are switched in 'amdpgu_dm'

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>