drm/omap: handle incompatible buffer stride and pixel size

omapdrm doesn't check if the pitch of the framebuffer and the color
format's bits-per-pixel are compatible. omapdss requires that the stride
of a buffer is an integer number of pixels

For example, when using modetest with a display that has x resolution of
1280, and using packed 24 RGB mode (3 bytes per pixel), modetest
allocates a buffer with a byte stride of 4 * 1280 = 5120. But 5120 / 3 =
1706.666... pixels, which causes wrong colors and a tilt on the screen.

Add a check into omapdrm to return an error if the user tries to use
such a combination.

Note: this is not a HW requirement at least for non-rotation use cases,
but a SW driver requirement. In the future we should study if also
rotation use cases are fine with any stride size, and if so, change the
driver to allow these strides.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>