Minor refactoring in encodeintra.c.

Merge code blocks for different transform sizes; use MACROBLOCKD as a
temp variable where that leads to smaller overall source code; remove
duplicate code under #if CONFIG_HYBRIDTRANSFORM/#else blocks. Some style
changes to make it follow the style guide a little better.

Change-Id: I1870a06dae298243db46e14c6729c96c66196525