msm: msm_fb: validating backlight level and scale value

Validating the value of backlight to be not more than the
max value and also ensuring that the backlight scale value
is not more than 1024, thereby ensuring that backlight
scaling factor is not more than 1

Change-Id: I6a6f4f0f0a8c272edcac5078276fb406ddee3e90
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>