drm/amd/display: Fix new stream count check in dc_add_stream_to_ctx

[Why]
The previous code could allow through attempts to enable more streams
than there are timing generators, in designs where the number of pipes
is greater than the number of timing generators.

[How]
Compare the new stream count to the resource pool's timing generator
count, instead of its pipe count.  Also correct a typo in the error
message.

Signed-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>