drm/amd/display: Increase scratch buffer size

[Why]
Larger data blocks are expected to be transferred between driver and FW
in the future.

[How]
Embiggen the scratch buffer to a cromulent size.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>