Fix uninitialized use of local variable pitch in radeon_setup_kernel_mem

Fixes server reset.

Pointed out by clang:

../../src/radeon_kms.c:2721:9: warning: variable 'pitch' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    if (!info->front_buffer) {
        ^~~~~~~~~~~~~~~~~~~
../../src/radeon_kms.c:2765:27: note: uninitialized use occurs here
    pScrn->displayWidth = pitch / cpp;
                          ^~~~~