drm/lima: Reduce number of PTR_ERR() calls

Store the PTR_ERR() result in local variable in clock init error path.
This makes the code consistent with similar section in regulator init
code.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621162117.22533-3-krzk@kernel.org