drm/amd/display: Revert DCN4 max buffered cursor size to 64

drm/amd/display: Revert DCN4 max buffered cursor size to 64

[Why & How]
The buffered cursor cap is expressed assuming a square cursor, and usage
of the cursor buffer is limited by the request size. For greater than 32
pixels, the request size is fixed at 256 bytes, so the maximum width
must be floored to the nearest 256th byte. At 4bpp this means even with
24kB DCN4 can only hold a 64x64 cursor in the buffer as even 65 pixels
would require 512 bytes per line instead of 256.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>