input: cyttsp-i2c: Don't set mode on unsettable regulators

Currently during boot, and every suspend and resume, we call
regulator_set_optimum_mode on every touchscreen regulator. However,
some regulators don't support this function, and this will cause
set_optimum_mode to print an error when this occurs. Correct this by
checking if it is valid to call set_optimum_mode on a given regulator
before doing so.

Change-Id: I71f0f14b16d9f333bd409d9d5150f11a7ccd8fdf
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>