Memory mapped IO was only accessed by MOVX instructions.

Look on the timing requirements of the LCD and of your 8051 derivate. Since on fast 8051 derivates it can be needed, that you must enable the memory cycle stretching feature.

If /RD and /WR are low active, then it can be direct connected to /RD and /WR of the 8051.
Otherwise insert an inverter (74HC04).

Prior longer times without LCD access you must write "mov p0,#0" to avoid floating data lines.

Peter