Extract out a MatrixMatrixBlockPanelEmitter::Dimensions struct; NFC

This gives me a convenient place to note that the m/k/n here are not the m/k/n
for the entire GEMM.  I didn't rename m/k/n to mc/kc/nr since the latter seems
somewhat redundant to me -- we could read 'c as 'column' and 'r' as 'row', but
that's the only possibility?

This refactoring will also be useful when implementing GEPP on top of GEBP.

PiperOrigin-RevId: 197474137