drm/msm/sde: add new perf tune mode for fix clk/bw voting

Curent performance tune mode supports normal voting and
minimum bus voting mode, which can be selected by writing
0 and 1 to debugfs core_perf/perf_mode respectively.

Add a new perf tune mode, i.e. 2, to select fix clk & bus
voting provided via debugfs node core_perf/fix_core_clk_rate,
core_perf/fix_core_ab_vote and core_perf/fix_core_ib_vote.

CRs-Fixed: 2009714
Change-Id: I87978e7c9ec43b691a75f120f8078a089ac711c0
Signed-off-by: Alan Kwong <akwong@codeaurora.org>