drm/msm: Simplify NULL checking in msm_disp_state_dump_regs()

The msm_disp_state_dump_regs():

- Doesn't allocate if the caller already allocated. ...but there's one
  caller and it doesn't allocate so we don't need this check.
- Checks for allocation failure over and over even though it could
  just do it once right after the allocation.

Clean this up.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/619660/
Link: https://lore.kernel.org/r/20241014093605.3.I66049c2c17bd82767661f0ecd741b20453da02b2@changeid


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>