drm/msm/gpu: check legacy clk names in get_clocks()

Otherwise if someone was using old bindings with "core_clk" instead of
"core" as the clock name, we'd never find it and gpu would be stuck at
27MHz (or whatever it's slowest rate is).

Fixes: 98db803f ("msm/drm: gpu: Dynamically locate the clocks from the device tree")
Signed-off-by: Rob Clark <robdclark@gmail.com>