Fix misleading indentation in sys/dev/drm2/radeon/evergreen.c

This fixes the following warning from recent versions of clang and gcc:

sys/dev/drm2/radeon/evergreen.c:554:4: error: misleading indentation; statement is not part of the previous 'if' [-Werror,-Wmisleading-indentation]

Direct commit to stable/{10,11,12}, since this driver has been removed
from FreeBSD 13.