drm/amdgpu: Handle 64-bit return from drm_crtc_vblank_count()

570e8696 ("drm: Widen vblank count to 64-bits [v3]") changed the
return type for drm_crtc_vblank_count() to u64. This could cause
potential problems if the return value is used in arithmetic operations
with a 32-bit reference HW vblank count. Explicitly typecasting this down
to u32 either fixes a potential problem or serves to add clarity in case
the typecasting was implicitly done.

Cc: Keith Packard <keithp@keithp.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com> for both this patch
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-4-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/amd/amdgpu/amdgpu_display.c

@@ -207,7 +207,7 @@ int amdgpu_crtc_page_flip_target(struct drm_crtc *crtc,
 	amdgpu_bo_unreserve(new_abo);
 
 	work->base = base;
-	work->target_vblank = target - drm_crtc_vblank_count(crtc) +
+	work->target_vblank = target - (uint32_t)drm_crtc_vblank_count(crtc) +
 		amdgpu_get_vblank_counter_kms(dev, work->crtc_id);
 
 	/* we borrow the event spin lock for protecting flip_wrok */

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3836,7 +3836,7 @@ static void amdgpu_dm_do_flip(struct drm_crtc *crtc,
 
 
 	/* Prepare wait for target vblank early - before the fence-waits */
-	target_vblank = target - drm_crtc_vblank_count(crtc) +
+	target_vblank = target - (uint32_t)drm_crtc_vblank_count(crtc) +
 			amdgpu_get_vblank_counter_kms(crtc->dev, acrtc->crtc_id);
 
 	/* TODO This might fail and hence better not used, wait
@@ -3982,7 +3982,7 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 			amdgpu_dm_do_flip(
 				crtc,
 				fb,
-				drm_crtc_vblank_count(crtc) + *wait_for_vblank,
+				(uint32_t)drm_crtc_vblank_count(crtc) + *wait_for_vblank,
 				dm_state->context);
 		}