drm/radeon/radeon_display: Decrease the size of allocated memory

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1] [2].

In this case, the memory allocated to store RADEONFB_CONN_LIMIT pointers
to "drm_connector" structures can be avoided. This is because this
memory area is never accessed.

Also, in the kzalloc function, it is preferred to use sizeof(*pointer)
instead of sizeof(type) due to the type of the variable can change and
one needs not change the former (unlike the latter).

At the same time take advantage to remove the "#if 0" block, the code
where the removed memory area was accessed, and the RADEONFB_CONN_LIMIT
constant due to now is never used.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160

 [2]
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>