drm/amdgpu: Fix video caps for H264 and HEVC encode maximum size

H264 supports 4096x4096 starting from Polaris.
HEVC also supports 4096x4096, with VCN 3 and newer 8192x4352
is supported.

Signed-off-by: David Rosca <david.rosca@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>