drm/msm: Remove 'src_clk' from adreno configuration

The adreno code inherited a silly workaround from downstream
from the bad old days before decent clock control. grp_clk[0]
(named 'src_clk') doesn't actually exist - it was used as a proxy
for whatever the core clock actually was (usually 'core_clk').

All targets should be able to correctly request 'core_clk' and
get the right thing back so zap the anachronism and directly
use grp_clk[0] to control the clock rate.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>