drm/amd/amdgpu/amdgpu_drv.c: Replace drm_modeset_lock_all with drm_modeset_lock

drm_modeset_lock_all() is not needed here, so it is replaced with
drm_modeset_lock(). The crtc list around which we are looping never
changes, therefore the only lock we need is to protect access to
crtc->state.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>