grvga: Fix error handling issues

This patch fixes two problems with the error handling in the
grvga_probe function and simplifies it making the code
easier to read.

- If the call to grvga_parse_custom on line 370 fails we use
  the wrong label so that release_mem_region will be called
  without a call to request_mem_region being made.

- If the call to ioremap on line 436 fails we should not try
  to call iounmap in the error handling code.

This patch introduces the following changes:

- Converts request_mem_region into its devm_ equivalent
  which simplifies the otherwise messy clean up code.

- Changes the labels for correct error handling and their
  names to make the code easier to read.

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>