msm: kgsl: fix mem leak in adreno_read_speed_bin()

The nvmem_cell_read() allocates a buffer which needs to be
freed by the consumer after use. This patch fixes this memory
leak.

Change-Id: I4ee81880f449288be1c82498bf460cdc17ab10a8
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>