msm: kgsl: Improve rGMU firmware loading time

Use gmu_core_blkwrite() for block writes to an I/O mapped memory,
to improve firmware loading time.

Profiled data of a6xx_rgmu_fw_start() over 500 iterations:
Old:- min:1071us max:8159us mean:1607us.
New:- min:182us max:646us mean:210us.

Change-Id: I80a1ae020b7669f4cbd41028d06babf5ad749473
Signed-off-by: Sravankumar bijili <csbijil@codeaurora.org>