drm/amd/display: Explicitly extend unsigned 16 bit to 64 bit

Coverity reports sign extention defects as below:

Suspicious implicit sign extension: mode->htotal with type u16 ... to
int (32 bits, signed), then sign-extended to type unsigned long
(64 bits, unsigned). If mode->htotal * mode->vtotal is greater than
0x7FFFFFFF, the upper bits of the result will all be 1.

Cast it to unsigned long to avoid possible overflow.

This fixes 4 SIGN_EXTENSION issues reported by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -977,8 +977,8 @@ static void amdgpu_dm_fbc_init(struct drm_connector *connector)
 
 
 	list_for_each_entry(mode, &connector->modes, head) {
-		if (max_size < mode->htotal * mode->vtotal)
-			max_size = mode->htotal * mode->vtotal;
+		if (max_size < (unsigned long) mode->htotal * mode->vtotal)
+			max_size = (unsigned long) mode->htotal * mode->vtotal;
 	}
 
 	if (max_size) {