drm/drm_mm.h: Fix the name of the referenced function in comment

drm_mm_insert_node_generic() is a simplified version of
drm_mm_insert_node_in_range(), update comment to reflect correct
function name.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101140445.2798-1-Liviu.Dudau@arm.com