1) Make a subroutine that outputs one character to the display chip
2) Make another subroutine that rewinds the chip address counter (You already have this one)
3) Make yet another subroutine that actually clears the display by writing non visible character to all it's lenght
4) Make a subroutine that outputs a string to the display

After this Your main code would look like:

static void WriteDisplayString( const char * aString ) {
    ResetDisplayPointer();
    while ( *aString ) WriteLedDisplayChar( *(aString++) );
}

static void ClearLedDisplay() {
   unsigned char ix;
   ResetDisplayPointer();
   for (ix=0; ix < DISPLAY_LENGTH; ix++ ) {
        WriteLedDisplayChar( INVISIBLE_CHAR );
   }

}

extern void main() {
    ClearLedDisplay();
    WriteLedDisplayString( "WHOAA" );
}

Those will not make Your code any more efficient or smaller but they make it very much more readable.