fbdev: msm: enable compilation of fb driver

Fix compilation error related explicit type cast error with
IS_ERR_VALUE macro.
Update CONFIG_MSM_BUS_SCALING to CONFIG_QCOM_BUS_SCALING.
Replaced read_write_v_h macro with function.

Change-Id: I748756e05f1e224e09f8131183d36cdbae628b74
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>