st/omx: Fix port format enumeration

st/omx: Fix port format enumeration

From OpenMAX IL section 4.3.5:
"The value of nIndex is the range 0 to N-1, where N is the number of
formats supported by the port.  There is no need for the port to
report N, as the caller can determine N by enumerating all the
formats supported by the port.  Each port shall support at least one
format.  If there are no more formats, OMX_GetParameter returns
OMX_ErrorNoMore (i.e., nIndex is supplied where the value is N or
greater)."

Only one format is supported, so N = 1 and OMX_ErrorNoMore should be
returned if nIndex >= 1.  The previous code here would return the
same format for all values of nIndex, resulting in an infinite loop
when a client attempts to enumerate all formats.

Signed-off-by: Mark Thompson <sw@jkqxz.net>
Reviewed-by: Christian König <christian.koenig@amd.com>